I know this has been on here before, but I would like some input from owners. Which is better for MX and which is faster? Any help is appreciated.

 

You said MX. Do you mean you want to take it on a Race Track? If so I would not go with a Scrambler. Look into a 400EX,DS650,Raptor or a CANNONDALE
If you want a great Trail/Dune bike than the Scambler is for you.

The chassis between the two are the same (400 has some extra Aluminum Parts (Wheels, Running Boards, Frt Bumper.)
The main difference is 2 or 4 stroke. The other factor will be if you plan to Mod it at all.
If you plan to mod it the 2 stroke will give you a lot more power for you money. You just can't beat a modded 2 stroke. Add a Pipe and clutch kit and the 500 will not touch it.
IF you plan to leave it mostly stock I would go with the 500. The 500 seems to have more low end grunt, witch is nice for tight trails.
They are both great bikes, It all depends what you plan to do with them. If you are thinking 400 this may be the last year you will be able to get a new one. The 2 stokes seam to be getting phased out. I have heard with the addition of POLARIS new real SPORT QUAD (LATE SUMMER), They will elimate the 400.

 

Buba, like your sig says, buy american!
Catfish, do you really want to compete in MX, or just hit the track once in awhile. A Cannibal would be a great choice too.
If you want a Polaris, the 400 would probably be better, but not by much really. There are a couple guys on here that have super sweet 400's that can compete in MX. I'd talk to them about their combos and see how competitive they really can be.

 

Catfish33,

By the time you modified a Polaris to be competative in MX, you could have bought the Cannondale Moto ($12,000). But that pretty much goes for just about any stock machine (except for Cannondale of course). Frankly, they are hard to beat as XC (cross country) machines.