There were stark differences in how easy the two styles of covers were to use. The stretch covers were particularly difficult to wrangle onto bowls of all types for three reasons. First, each cover expands widely from its initial diameter, so selecting the right size from the pile was a guessing game. It often took us several tries to find the stretch cover that fit a given bowl. Second, holding the stretch covers in place on one area of a bowl’s rim while trying to cover the other edges was difficult—despite small tabs on the covers’ perimeters meant to assist in stretching. We didn’t have enough hands to both brace and stretch at the same time. We found ourselves having to brace the bowl against our torsos and stretch with both hands, worrying about spilling food the whole time. Even once we thought we had a good seal, the covers would sometimes pop off a few seconds later. Two of the stretch cover sets were especially hard to stretch and prone to slipping off the bowls’ rims; the remaining set had a bit more give, making it slightly easier to stretch and hold in place. Finally, the sizes in each set weren’t always compatible with the standard-size bowls in our kitchen. Two of the sets didn’t have an option large enough to cover the frequently used 5-quart bowl from our winning set of metal mixing bowls.

In contrast, the flat covers couldn’t have been easier to use. The covers in two of the sets were almost perfectly round while those from the remaining set were shaped like lily pads; all three of the sets had central handles that made them easy to grip and move on or off containers. To seal a bowl, we simply set the most appropriately-sized cover right on the rim and gave it a slight push in the center. The covers would settle into the bowls as we pushed, letting out excess air, and then spring back slightly to form tight seals that were often strong enough for us to briefly pick up the bowls only by the covers’ handles (though manufacturers do not recommend this). To break the seals, we peeled the covers’ edges back from the bowls’ rims, then simply lifted them off. For the most part, matching the flat covers to the bowls was much easier: Covers that either matched bowls’ diameters perfectly or were moderately larger in diameter than the bowls both created tight seals. And two of the flat cover sets had options large enough to cover our largest mixing bowl. When it came to ease of use, the flat covers won out over the stretch covers every time.

We continued to notice differences between the two cover styles as we evaluated seal strength. Once we finally managed to muscle the stretch covers onto bowls filled with fruit salad, we stored them in the refrigerator for three days, along with a bowl of fruit salad we covered with plastic wrap for comparison. Two stretch covers stayed on and kept the fruit just as fresh as the plastic wrap did, but the remaining stretch cover frequently popped off throughout storage, much to our frustration. As a result, the fruit in that cover’s bowl was a bit drier than the other samples after three days. Still, every stretch cover formed tight seals over bowls filled with both grapes and water, which stayed in place when we shook the bowls or tilted them over the sink.

The suction-style covers didn't seal well when we added some motion to the mix; when we filled bowls with grapes, sealed the lids on top, and shook the bowls, the suction lids didn't hold their seals and grapes flew out. Ultimately we still like this style, but if you're using the lids to transport food, they do require extra care. 

Conversely, all the flat covers were able to strongly seal bowls made of every material we tried, including metal, glass, plastic, ceramic, and wood. They preserved fruit salad as well as plastic wrap over the course of three days, since they remained tightly in place throughout the refrigeration test. However, we did have some trouble with the flat covers during the grape and water tests. Their seals eventually broke, sending grapes flying or water dribbling into the sink. But we like the flat covers for their aptitude where it counts; their ability to easily store food in the refrigerator or cover it on the go makes them a great storage option. 

The manufacturers of all the covers in our lineup recommend them for use in the microwave as covers for heating leftovers or steaming vegetables. All the stretch covers are advertised as being microwave-safe, but one company warned against using its covers to completely seal bowls while microwaving. We found out why: When we used the stretch covers to seal glass bowls filled with water and then heated the water in the microwave to boiling, expanding steam caused the stretch covers to pop off. Leaving the bowls slightly uncovered wasn’t feasible because the lids only stayed in place when they were fully stretched around the bowls’ rims. The flat covers performed better. As the water boiled, steam escaped through tiny openings that formed around the rims, but they contained most of the steam and hot air well—much better than the exploding stretch-style covers. We still think the primary use of these covers is food storage, but the flat covers’ usefulness when microwaving was a plus. 

We were interested in whether the covers would retain stains and odors and how they would hold up over time, so we conducted a few durability tests. First, we spattered the covers with measured amounts of ketchup, mustard, and olive oil, and then we let them sit overnight and washed them the next day. When we smelled them, we noticed that all the covers retained strong odors. The three stretch covers and one of the flat covers also held on to mustard stains. However, every cover’s stains and odors faded with additional washings. Second, all the covers are dishwasher-safe, so we washed them another 10 times in the dishwasher. Each cover survived unscathed, without warping or deterioration. Finally, we stretched one cover from each set to its limits 50 times before checking for any signs of damage. Five of the six covers perfectly retained their shape and structural integrity, but one lackluster stretch cover warped the slightest bit. It was harder to secure that cover over a bowl due to its altered shape.

In the end, we weren’t impressed with the stretch covers, and we can recommend only one set with reservations. We had trouble deciding between two flat covers when naming a winner, so we compared their performance in one more test. People sometimes use silicone flat covers to act as lids for pots and pans while cooking, so we used both covers as we steamed broccoli for Beef and Broccoli Stir-Fry. The scalloped edges of the lily pad–shaped cover didn’t fit our 12-inch skillet perfectly, letting steam escape. A model from GIR performed the best: Its nearly round shape covered our winning 12-inch skillet well, and it trapped the perfect amount of steam to cook the broccoli to an ideal crisp-tender texture in only a few minutes. It works well on most 12-inch skillets, making it an acceptable substitute for our winning 12-inch skillet lid. The GIR Suction Lids made strong suction seals on bowls of a variety of materials and sizes, but they're not infallible: much like plastic wrap, if you cover a bowl with one of these lids and shake it hard enough, the seal will break. Still, under normal use, they excelled, preserving fruit salad as well as plastic wrap, resisting stains, holding up to repeated washings and 50 vigorous stretches without any signs of deterioration, and even containing steam well in the microwave. These covers would be a great addition to your food storage lineup.