I tried asking this over at RepRap.org but was ignored, so I'll try again here...

I've been looking at LCD displays on eBays and am kinda lost. Too many options, too many slight variations. Can I get some advice on what I should be looking for, please? Are there any specific versions that are better, or should be avoided? What about Form Factor (Some are taller and have the control knob on the bottom, some are wider with the knob on the side)?