Here's a bit of specific info about heirlooms which may or may not be relevant to you (depending on where you live.) Here in western N.C. there are a LOT of small farms growing these tomatoes now and they don't charge much for them either so you get to taste a lot of them. A couple of examples:
German Stripey: Yellow with red and orange stripes, inside and out. Can be really misshapen but incredibly good. Very low acid. They look great in a salad, too.
Pink Brandywine: This one came out of Amish country. Dark red/pink. More acidic than the Stripey. Close to being the perfect tomato IMHO.
Cherokee Purple: When these are ripe they really are close to purple. More sweet than acidic.
Mortgage Lifter: I love the name of this one. People planted these because of the yield per plant. These suckers can get up to 3 pounds. Very close to a beefsteak but there's more happening in the flavor. You'd have to compare them to see what I mean. They're kind of a flat shape which means that a single slice can cover a whole piece of bread.
Persimmon: This is one gorgeous tomato. Pink orange like a persimmon. Meaty rather than really juicy with a really fruity taste. Almost an apple flavor to it.
Aunt Ruby's German Green. This one stays green when it's ripe. It's got a tang to it, almost citrusy. Great to use in place of those green/red tomatoes you get in Italy for salads. Also, it's cool to have an all green salad with tomatoes in it.
This can go on and on. If you love tomatoes you can really get knocked out by some of these. That's not to say there's anything wrong with a perfectly grown beefsteak from Jersey. Main thing is you can't get good ones in either supermarkets or from regular suppliers. And, of course, the farther you get away from big cities, the cheaper and more plentiful these things get.
If you want to see a great list with some pix and descriptions check this out: http://www.appalachianseeds.com/tomato-plants/tomato-plant-catalog
I love tomato season. We eat them every day for breakfast. I weep when the season is over.