I have an 08 525XC and my buddy Driver has a 09505 SX, we just got off the dyno with his bike, and mine has lots of dyno runs on it, I like the SX, here we can get an 08XC 525 for $7600, and the SX is $10600.00 the SX is better set up for MX, but 3k more in dollars is alot, both make close to the same power my XC stock w/free mods made 49.5hp and 39.1 ft lbs, the SX made a tad more hp and a tad less ft lbs, the power curve is better on the SX also your choice, I know i'm no help
I think a 450 xc with 8/06 cam, free mods (exhaust baffle removal and cut out some holes in the airbox) will run with any 450 sx. Buy the a arms, Have Ron valve the shocks and I think you could build a great machine for less money than the SX. Remember the SX needs shock work before you can be competitive on a track at most levels.
I own an '09 505 SX. The shocks on the SX are very stiff. I guess it depends on your weight and racing experience. If you are more of a beginner the power of the XC comes on easier. If you have been racing for a while and really get "on it" you may want the SX. Either way you go you will have to re-do the rear shock for your weight and level....not having any luck with the fronts due to the odd size of the springs (2" id is common, 1.88" is what the WP shocks have). I am sorry I sound kinda cryptic but it really does depend on alot of different factors.