Imho Samui aint a patch on Phuket when it comes to girls nightlife and tourists.
Green Mango has about 30 bars and 1 end is aimed at young backpacker types (and there are shitloads of them) the other end is girlie bars but there is just not the same kinda atmosphere you get down Bangla.
We saw a lot of young guys being very disrespectful towards bargirls and my wife and I hated that.
The only highlight for us was watching 2 gobby Aussies getting a hammering from an English bar owner, strange nobody saw anything when the police arrived lol and me chasing a guy for his unpaid bar tab earning us lots of free drinks at that bar every night.
If you want girls, nightlife and friendly tourists go to Phuket, if you want drugs, a younger tourist and more low key girlie scene go to Samui.
The beach is nice though
Dave