Sorry if I have repeated what others have said previously.
Just got back from a bucks trip in coughet, Koh Samui and Koh Phangan about 3 weeks ago.
coughet beach isn't that great along Patong. During the day it's not bad, not seedy, but along Bangala road at night it will get very seedy, but fun. Saw heaps of families walking up and down that road at night. I think all the seedy promoters stay away from families. However, I'd skip this place if you have already been before. Not that interesting unless you want to party at night. However there's more day excursions/activities to do in coughet and cheaper comapred to Koh Samui.
Koh Samui: Stayed at OZO chaweng in the beach front room. Towards the south of Chaweng beach so it's a bit quieter. Our room was the closest to the beach and had amazing views just a few steps to the beach. Clean beach, good sand, clean water, family friendly, water park (
https://aquaparkchaweng.files.wordpress.com/2013/03/aquapark-catapult-bag-2.png). It was right next to our hotel. Lol, it looks awesome and fun while your laying on your beach chairs, but after 15mins (out of our 2hrs of play time) playing in there, it is so tiring.
The beach vendors don't bother you as much anymore unless you invite them over as there's security guards on the beach (part of the hotel) who tell them to keep walking so as not to disturb hotel guest. I'd go back with the missus to this hotel/beach. There's more to do here during the day; more restaurants/shops etc. compared to somewhere like Lamai Beach. There's also a brand new mall in Chaweng, which is massive considering Koh Samui isn't that big of an island. Good location if you want to relax but also be within walking distance to shops/restaurants etc.
Koh Phangan: We went for the full moon party and stayed 3 days. It probably isn't worth going since there's not much to do on the island but relax on the beach, but there's heaps of coral at Haad Yao beach, so not a great beach to swim in.