I am in the same situation. I am still strongly considering a KTM as my next quad. I also ride a little bit of everything from MX tracks to XC courses but mostly desert trails. From what I hear, the 525 almost has too much "kick" for the trails. The 450 is apparently easier to ride fast and more controllable. If you are looking into racing, make sure the classes are designated by age and/or experience and not by engine size because anything bigger than a 450cc usually gets bumped into the "Open" class where you'll be racing some outrageously modified quads and your 525 might not even be enough.
If I end up getting a KTM, I'm going to go with the 450XC.
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2004 Honda TRX400EX (RED)
PEP, Hydrodynamics, Lonestar Racing, Rath Racing, Pro Armor, Pro Taper, ASV, Streamline
2009 KTM 450SX ATV (ORANGE)
Fox Racing Shox, Lonestar Racing, Precision, Houser, Motoworks, Hiper, Fuel Customs