I'd look at a Sig P226 or possibly a Walther if you want a step up from the Glock. The XDM is an excellent gun at $500 new and $400 used, but at $600+ new? No way. Either stick with the Glock, or look for a true upgrade for the price (in my opinion).
IMO, the only thing youre gaining by going to a sig over a glock is a lighter wallet.