Java, I got this from America's Test Kitchen:
It turns out that what matters is size, at least with our pot roast recipe. We recommend buying a slow cooker with a minimum capacity of 6 quarts. Anything smaller and a modest 5-pound roast, pork loin, or brisket won't fit.
Shape also matters. We found the round crock styles to be deeper than the oval crocks, and they heated more evenly. That said, while the depth and shape of these round cookers made them perfect for submerging a roast in braising liquid, it proved a hindrance with recipes requiring bulky, layered ingredients, such as chicken parts or ribs. Oval-shaped slow cookers have more surface area for cooking and are better suited to these kinds of recipes. Because oval cookers also work when making chilis, stews, or roasts, they are the more versatile choice. However, if you're going to use your slow cooker only for stews and chilis, a round cooker is a possible option...
One new feature proved less desirable. Two of our models claimed to have stovetop-safe inserts that could be used to first brown the meat and then put directly into the slow cooker. We found, though, that neither insert browned meat very well—the recommended maximum medium heat simply doesn't get the job done...
All-Clad Stainless Steel Slow Cooker with Ceramic Insert: This cooker aced the evaporation test and all the cooking tests, and it has every feature we want, including insert handles and a clear lid. Provided a steady, slow heat that is ideal for breaking down the collagen in tough cuts of meat without overcooking them.
KitchenAid Stainless Steel Slow Cooker: This model cooked slightly hotter than the other contenders, but none of the finished food suffered as a result. The slightly squared insert shape was praised for being "easy to pour out of."
Cuisinart Slow Cooker: This machine did very well in all cooking tests. Several testers were surprised that the bulky, boxy exterior of this cooker got very hot during long cooking. "It looks like it should be insulated," said one.
Hamilton Beach Programmable Slow Cooker with Temperature Probe: While the temperature probe itself wasn't a lure for testers ("slow cooking shouldn't be that temperature-specific"), this model passed the evaporation test and performed well in the cooking tests. This cooker was downgraded for being the only one without handles on the insert, which made removing it difficult—especially when hot.
Rival Oval VersaWare Crock Pot: "No timer?" asked testers. "No 'on' light?" The opaque lid was another strike against this cooker, but it did perform well in the cooking tests. Several testers complained about the "overhanging lip" on the insert, which necessitates complete inversion to pour out contents.
Hamilton Beach Oval Stay or Go Slow Cooker: The gimmicky travel clips and recipe name tag were not part of this model's appeal, but testers did appreciate its solid performance in the kitchen and the "straightforward, no frills" ease of operation. This inexpensive cooker does not have a timer, which is a serious drawback.
West Bend Oval Versatility Slow Cooker: Testers complained about the lack of an "on" light, especially since this machine begins heating as soon as it's plugged in-there is no "off" setting. This model was also downgraded for heating faster on "low" than "high" and for having an awkward-fitting lid that "falls into the pot easily."