It's all about preference, if you want a screen, or if you want a tone, if your a land detectorist, or a water detectorist, if you're in ore enriched ground, or dirt. There is many types.
I strongly suggest starting with a VLF detector. I've tried 10 different detectors and companies, and so far the very best detector I've found is the Tesoro Silver Umax it runs $232
All the owners of them will agree that it's amazing. Those who haven't tried it will knock it, because they like theirs, but from experience with many detectors, I can tell you this is one of the best low priced machines, and very well made. It also has the best discrimination against Iron that I've ever seen.