Yeah your original question here is incomplete and unanswerable if you don't specify what they are buying the radio for, bunch of people already touched on that.
AM is the all-time guaranteed fallback option that will be able to communicate with almost any other CB equipment, and is always available and affordable and easy to use. FM is a better choice if you are only doing a setup to talk to people you directly know (your family, your buddies, etc) but you must sell everyone in that group on investing in FM which might be a hard sell when perfectly good AM radios can can had so much cheaper. SSB is obviously a necessity if you are interested in skip (and you want to actually make DX contacts with low or normal/affordable power outputs).
Like was said before it all depends on what they are trying to accomplish. FM is a tough sell though because it costs more and takes more effort/understanding/intelligence/coordination to use with PL/DPL and you might get brand/model locked if it's not implemented by a common standard that all manufactures use (not sure if that comes up much or not). If you are only communicating locally with people who are savvy with the extra settings and setup it will be great but more likely if you are just out Jeepin with your buddies and a few randos anything more complicated than "turn it to channel X" is probably going to be a non-starter for quite a few... I've got better things to do than to spend half an hour going around trying to get everyone's radio on the right mode and tone just to find out the second-to-last guy just has a run-of-the-mill AM radio even though he said he had FM (He didn't actually have a clue what you were talking about when you asked earlier) so now you are going to go around and help everyone else get back into regular AM mode... nope, no thanks. Stick with what is simple, easy, affordable, and "just works" for the local group. Stick with SSB for DX. If I hit that rare scenario where I am only talking to one guy, and I'm trying to *only* talk to that one guy, and he is smart enough to know the difference and complete the right setup we agree to then sure, I'd try it for the heck of it. Still going to be extra setup and complexity for not enough gain, IMO.