All the Etymotic models are actually engineered to sound somewhat similar, as the founder of Etymotic is an audiologist who has tried to deliver what he has researched to be an accurate sound response to the average ear - all the models are deisgned to sound neutral rather than fun. At least that was the marketing says, but they have a very long pedigree in the market, and (I think?) were the first to actually popularise the idea of having in-ear's for non-professional use.
I haven't heard any of the models except the MC3/5, but I can tell you the technical differences:
The ER-4S is actually the flagship model, a single balanced armature with a very high impedance designed to be used with an amp. It's been around for ages but still gets plenty of respect around here. The ER-4P is the same as the ER-4S, but with a lower impedance. Apparently it doesn't have quite the same amazing clarity as the ER-4S does. The ER-4B is a bit of a weird one - as I understand it is the ER-4S but tuned so that there isn't a frequency dip around 10khz or something... something very specific to do with music mastering. I'm sure someone else could tell you about it.
The HF-5 /HF3 actually uses the same drivers as the ER-4's just in a different housing. Unlike the ER-4's, where each individual earphone is hand matched so that there is a balance between both left/right channels, the HF5's don't have that kind of quality control and thats why they are cheaper. Mind you, most manufacturers don't hand match, so this is no strike against the HF5's. If you ask Bennyboy around here, he will tell you how much he loves them.
The MC5's/MC3's are the new models Ety developed with dynamic drivers rather than balanced armatures, so that they could get the same sound at a cheaper price point. I can tell you that the MC3's are built fantastically, have a transparent sound which while it might not be fun, it is startlingly refreshing. They are great value and offer amazing sound isolation like the rest of the Etymotic line.
The signature aspects of the Etymotic can probably be summed up as, technical, neutral sounding earphones with class-leading noise isolation.
Anyway, thats as much as I've been able to gather.
Btw, welcome to Head-fi!