I've stayed at the Sofitel Asoke Residences, Grand Sukhumvit by Sofitel, as well as a few other Sofitels in Bangkok. I prefer the Grand Sukhumvit personally and liked the Asoke Residences the least. It was too far of a walk in the heat, though in December the weather should be alright and I just didn't like the feel of the place. The Grand Sukhumvit is closer to the BTS and an acceptable walk in my opinion though sometimes can be very noisy with lots of uncontrolled kids in the lobby and elevators. It bugs me a bit but once in the room all is fine.

If you're going and coming a lot I think you'll tire of the walk from Asoke Residences to the BTS. Once a day might be ok.