Tough to say. Since it has the same two drivers, it will be literally identical to your mains, which ultimately would probably yeild the best results, especially if they are all mounted vertically. The benefit of a MTM center would likely be a slight bump increase in midrange and sensitivity (sensitivity won't matter much once calibrated though). Whether or not the 600 center would sound "better" is hard to say without listening to both. Me personally, my speakers are all identical (but they are all MTMs too) and there's a lot to be said for a setup like that IMHO, so I'd try it first with one of the 602s and see how you like it.