Sunny Beach is the larger (indeed, the largest on the Bulgarian coast) with all that implies - more hotels, more bars, more restaurants, more people. It is also mainly on the flat, whereas Golden Sands is quite hilly in places, and has a smaller beachfront; there's really not a lot between the 2 resorts, except for size.
I personally find the countryside around Varna/GS/Albena to be more interesting than the Bourgas/SB area, but that's just my preference.

I have to say I agree with doonhamer32. Personally I feel Sunny Beach looks too much like the Costas although Nessebaur is pretty, but that is just my personal preference. St. Konstantin is my favourite place of the resorts but that is because I prefer smaller and quieter places.