If your decent at UMK3 but new to arcade sticks, it'll be a massive downgrade in your gameplay for a while. For how long? That all depends on how dedicated you are and how many hours you put in on the stick. I switched over from a ps2 controller to stick last July, I had to play all day for like 3 days to even be able to halfway compete online again. Then I focused all my energy learning one character (Kung Lao). I'd say, 3 - 4 weeks doing this, my Kung Lao was as good as it ever was on the ps2 controller. Now, I couldn't use Kung Lao on a ps2 controller again and I wouldn't want to. The great thing about using a stick (for me) is, my hands never get sore from playing too long, on the ps2 controller my thumb would be hurting after a few hours of using Lao, from doing all those spins. I think the newer ps2 controllers have a stiffer D-pad, this could have been the problem.
So now after 8 months, the only characters I'm still better with on the ps2 pad are Kano, Liu Kang, and Jax. The reasoning is cause on the ps2 pad, I'd assign 2 LK buttons so I could buffer the cannon ball, bicycle kick, or ground slam for these characters easy while playing. Eventually it won't matter.
If you want to buy one, i am gonna sell my PDP MK9 stick for PS3 and PDP Klassic stick for 360. Get at me.
I hope your saving up some pennies. lol