In terms of power, I never really found my P450 'wanting', it could go from 0 to license-worrying in no time at all, but it always felt 'flat' for want of a better word. My particular car dyno'd at a smidge over 500hp before the remap, and figures upwards of 480 were apparently very common, so the performance gap might be smaller than you expect.
Whichever you choose, you're going to get a car that isn't short on performance. If you're buying on that alone, get the 575. If you don't mind sacrificing a bit of grunt but want the rest of the 'theatre', grab the P450 and put the saved money aside to pay for some fantastic road trips.