Both are great, I personally loved 09 myself, but 10 was a needed fix to some of the problems 9 had. Outside of the extreme update in graphics (I mean you can literally see the sweat on these guys in 10), one of their biggest fixes was the choppy transitioning in take downs 09 had. 09 it would be grab, pause, take down, pause, transition, pause, transition, pause, etc. 10 took it and made it so much more fluid and smooth. Other than that all the game modes are the same for the most part, and a lot of fun.
My only beef with 10 was I thought they took out a lot of the knock out power 09 had in it. I mean some of the lates and greats could still drop you with one good hit, but unlike 09 my custom fighter just didn't feel as strong as before (I'm a stand up fighter in those games, so I always go for heavy punching and take down defense) Other than that, I would honestly say get both, for 30 bucks for both games it's definitely worth it.