As a Platinum member you should get upgraded to a Club room at Sofitel Sukhumvit, with lounge access, free breakfast and free internet, at least I was always upgraded in the past. Plus the location of the hotel is better than the the others location, it's near BTS and MRT station, plus it's near TErminal 21 shopping mall.
Upgrades at Sofitel So are a little bit more difficult. They will upgrade you if you book a Club room or higher, but if you just book a So Cosy or So Comfy room it can be hit and miss. Plus imho the location is atrocious.