Look for one where they identify the bulb manufacturer within the lamp housing. If the bulb manufacturer is either Philips, Osram, Ushio, or Phoenix, you will be getting a lamp with at least a decent quality of bulb in it.
If you buy a spare, make sure you use it for a few hours before storing it. Some lamps, especially compatibles, will arrive dead on arrival. Since the warranty period will usually last 90 or 180 days (1 year at most) you want to make sure all of the lamps you buy are not DOA.