I nearly always stay at the SGS and always with hand luggage only but I would not consider using the Rail Link, it just isn't worth it imho.
I can't remember it taking more than 40/50 minutes from Suvarnabhumi to the SGS door to door in what must be around 15 stays since the former opened, and going back it's usually less than 30 minutes.
The only slight PITA is if they come up the WGS side of Sukhumvit Road and have to u-turn at Asok. A few weeks ago I timed it at 22 minutes from getting in the car at Suvarnabhumi to being stationary outside the WGS. Of course it was another ten minutes before I was at check-in at the SGS due to the u-turn and the crazy traffic lights system.