I'll start by saying that I've just finished my AS year of sixth form. I, like you, didn't try particular hard at GCSE and, although my grades in the first instance were higher, it is definitely possible to get more from sixth form as you expect from yourself.
In regards to your sixth form and college offers I would suggest that the college would give you the greater flexibility-the subjects are just that bit more academically rigorous, thus gaining you a little more prestige which could be all important in a uni application if that's the route you choose.
You do mention the travel though and I think that is actually the biggest factor in your decision. It being 80 minutes away means you'll be spending 2 hours 40 minutes travelling each day. Considering that you will have much more work than at GCSE it may be that travelling so far removes your opportunity to complete this. I personally go to a grammar school sixth form which attracts people from a wide area, but yet I know no one who travels so far.
I think, bearing that in mind, I'd chose the sixth form. 80 minutes each way is, in my opinion. is too much travelling. Maybe it'd be worth your time to try and get the same subjects at the sixth form as you would get at the college-I'd certainly recommend that approach.