I liked the world of skate 2 a hell of a lot more than 3, but 3 has a lot of quality of life improvements.
Free skate is available in both in the areas of the game. 3 lets you create your own parks and download other people's parks*. Replayability is the same, you play as much as you feel satisfied trying new tricks/lines/etc.
The gameplay, with "hardcore mode", is a bit more solid in 3 as expected. Both are fun if it's your type of game.
*They shut down the skate.reel site for 3 in April, and I think 2s has been down awhile, so I'm not sure if you can still download parks or not. If I think about it, I'll boot up the game and check.