Really, when it comes to deciding what to grab at the grocery store, it all depends on what you're making, and -- of course -- on your personal preference. For example, if you're a person who's picky about texture, maybe you prefer crushed tomatoes to peeled tomatoes and that's a pretty easy swap to make. Tomato sauce, tomato paste and tomato puree all have different textures, too, so you may fancy the feel of one above the other.
If you want to make delicious stuffed tomatoes, you'll obviously need to start with whole ones and cut the top portions of the tomatoes off. If you want to put some tomato on a sandwich, however, you'd look pretty silly using a tomato in its entirety or dicing it up and trying to keep all the bits from falling out. Slicing it is probably the best way to go in the sandwich arena. While preparing Pico de gallo, you may want to finely chop your tomatoes, but for chunky salsa, using diced ones works just fine.
Ripeness is another factor you'll want to keep in mind. Homemade ketchup is more easily made with ripe, mushy tomatoes, but stuffed tomatoes should be nice and firm. There's also the matter of cooked versus raw tomatoes. By cooking tomatoes, you seriously up the level of great nutrients like lycopene, although you do sacrifice a bit of some of the others, such as vitamin C. This isn't too big of a deal though, since tomatoes come loaded with vitamin C in the first place. One cup of raw cherry tomatoes, for example, has 32 percent of the Percent Daily Value recommended (although it's important to keep in mind that's just an average; people's need for vitamin C varies) [source: Nutrition Data]. Also, canned tomato products generally count like cooked tomatoes because they're heated during processing.
On the next page is a bunch more information prone to please foodies looking to satisfy their hunger for knowledge.