Yep, I'd say that Napper and Star Hopper are likely the most popular because of their different hair colors. The others' only major difference is their cheek symbols, but those are really neat! Some of them also have differences in body tone, but there's a wide range they can be even within each release!
I've seen people charge a bit more for a complete set than the sum of the ponies individually since it's convenient to buy the whole set at once; it's up to you whether you do that, though. If I got them cheap, I'd sell them for a bit cheaper than the sum because that's a deal I'd love to find! That may just be me, though!