For the 7inch versions, specs are pretty much the same, except that the Tab 3 has been updated to 4.4.2, and AFAIK the Tab 2 had it last at 4.3. Also the screen is bigger. Well they're both 7inches, but the Tab 3 has no onscreen controls so you have full use of the screen.
However in terms of performance you won't see any difference. I'd stick with the Tab 2 since the benefits don't justify the cost. Unless perhaps you meant the 8inch version which has superior hardware. However, my recommendation would be to skip the 3rd generation and go for the Tab 4 instead. Whatever your usage, it will be a better use of money.