The Transit Pros worked out well for me and I think I could get by with just them. I prefer to use them with small front panniers though. I also like the Nashbar or Performance Waterproof Panniers.

You will probably find that the length of the trip has minimal effect on how much you carry. I carry the same stuff for a week as I did going coast to coast.

FWIW: I used them to go coast to coast and think they could make the trip many times over. I plan to do more long tours and do not plan to upgrade. I will use either 4 of the Nashbar Waterproof or the Transit Pros on the rear with the small Nashbar Waterproof on the front. I own both since I bought bags for the three of us for the TransAmerica.