For canning, I want one that has a deep red with a nice acidity and flavor and that yields mostly over a short period of time, and in my experience that means a smaller sized tomato. For quicker processing, it needs to be round or oval, very symetrical, with a shallow core (hate it when the tough green part extends too deep--a real pain to carve around, slows me down big-time.) I found a new variety this year that I am very pleased with for canning, called 'Abe Lincoln'. And it is O/P. It meets all of the requirements above, but still makes a nice size, about baseball sized or a little bigger and perfectly round.
Larger tomatoes usually end up only coming on one or two at a time, which means I have to put in more plants to get enough if I were going to can them. This is the case with 'Delicious', which has the deep red and the flavor I like in canner, but really is better as a slicer. If I have extra I will can them. And the larger varieties tend to have more cracks, deep lobes, cat-facing and deep, tough cores which help hold their weight on the plant. So I reserve those for the tomato "connoisseur" in me, enjoying them sliced fresh on the side or on a tomato sandwich. They tend to have more variety in their colors and flavor and often a lighter or sweeter flavor that is not really potent or acidic enough for canning stuff like spaghetti sauce, although you can still use them for plain ol' canned tomatoes for stews and whatnot, and they really need the lemon juice to make them acidic enough to safely waterbath.
One exception to the canning tomatoes would be the 'Costoluto Genovese' which are characteristically fluted, but since they are a paste tomato, I cook them with the skins on, then run them through the strainer. All the other canning tomatoes (not paste tomatoes specifically), I blanch and peel first, which is why I like them round and uniform.
