I just happen to be sitting here setting drag on my 500g and I dropped it down to 10lbs and it still loads pretty good. It seems perfect around 18-20lbs. Ive caught plenty of medium size snapper on it and it was still good for that. If you have much chance on fish over 30lbs I would stick with the 500g.
Are you fishing around rigs or open water? If rigs Id stick with the 500 if open maybe the 270 would work. Ive never used a 270g.