Samui has 2 bangla-road type areas, neither of which are of course anywhere near the size of the one in Phuket. If you know bangla road well then just pick 1 soi and imagine that's the only one. (And don't pick Soi Eric because the girls in Samui aren't as hot as the ones in Soi Eric).
In Chaweng there is Soi Green Mango which is around the green mango dance club, and quite a few beer bars and 1-2 gogos as well.
In Lamai there is a strip in the middle of town where there is nothing but beer bars. example: http://www.youtube.com/watch?v=7N1n6...eature=related
There is also another area with a Muay Thai boxing ring that is surrounded by another 10-15 beer bars. On saturday nights the girls from those 10-15 beer bars fight each other muay-thai style http://www.youtube.com/watch?v=eduf9dmP2Ek
(and also they put on some male fights featuring i guess 16-17yo fighters who can't get a gig in Samui's main Muay Thai arena).
I would estimate there are 40-50 beer bars in Lamai and a similar number in Chaweng. The ones in Lamai are all within walking distance to each other, same in Chaweng. The 2 beaches themselves though are about 20 minutes apart by car.
There is a Tesco Lotus complex in Samui which has a movie cinema and is about the size of your average Australian Mall, but it's not anywhere near as big as the megamalls of the US or the ones found in Phuket or Bangkok. http://www.photos-samui.com/tesco-lotus.htm
<---- bunch of photos of the beaces of Samui here. I think Lamai is the best beach. Chaweng beach is just as good but more crowded so I prefer Lamai. Either one will be crowded during peak season (nov-january) but Lamai is pretty deserted the rest of the year.
As far as Koh Phangan goes, only been there once which was for the full moon party. Not as many beer bars there, it's mostly geared towards backpackers and younger tourists in general. The main form of entertainment there is the full moon party which is a giant rave once per month which attracts anywhere from 5-10,000 people. Mainly european tourists.