There's not an aweful lot to it to be honest, especially when you're using a 203. There is some differrence in performance relatinng to the surface area of the rotor which actually contacts the pad surface. However, since you've got a 203 Avid it'd be the difference between 'shiiiiit that's strong' and 'fuuuuck that's strong' so not much in it . With regards the 200 rotor, don't forget that's the OD so you're actually only losing 1.5mm that the pad can grab and so in my mind a 200 rotor will be perfectly adequate. Basically, at the end of the day it comes down to aesthetics. Want a wavy rotor? Want a sawtooth rotor? Don't mind a round one? Just go with what you fancy but the price won't make much difference at all.