All I can say is that if I disable AppleTalk (the only Apple-related protocol this model supports) on the HP Jetdirect 175x print server, if I try to add the printer to my system preferences, there is no sign of it. Once I turn AppleTalk back on, the printer shows up.
On the en3700 print server, I can find the printer without AppleTalk being enabled, since it supports many other formats. As I indicated, though, this is a very expensive model (about 3 times the cost of an Airport Express). There is no way I could justify buying it new, but I managed to get a once-in-a-lifetime deal on a used one through craigslist, so I jumped on it. I don't know how well it would work on a non-HP printer, though (since I have an HP Laserjet, this is presently not a concern).
As to how many print servers out there will work, I have no idea, but it is probably best to buy locally, so that it is easy to drive back to the store to return it if you find it isn't Mac-compatible.
I stand by my opinion that, given the features it has, the Airport Express represents the best bang-for-the-buck out there (even if you buy new, and a definitely better deal if you can get a used or Apple-refurbished one). Plus, it is pretty much guaranteed that this will be Mac-compatible, and it will work with a variety of printer brands. Yes, you might save a few bucks using a different brand of print server, but what is your time worth trying to make it work (plus the time and gas involved in returning it if you can't make it function properly)?