I started out with an Arkel Tailrider on a rear rack and a small ditty bag in front. Switched to a Carradice Lowsaddle Longflap on a Bagman support, both to get more carrying capacity with lower weight, and because the Carradice moves the weight closer to your center of mass so it doesn't affect handling as much. Rode BMB with that setup. For PBP in 07, realized that the Carradice is designed to be used without a rack and ditched the Bagman. Bought a small handlebar bag. By BPB in 11, I had decided that I like having most of the things I need in the handlebar bag, so I bought a bigger handlebar bag (Gilles Berthoud 28) with a rack (and a bike with low-trail fork) and a smaller Carradice Pendle. For rides up to about 300km or even longer but with little change in expected weather, I can go with just the GB bag and maybe a small wedge pack for inner tube, tools, and parts. If there is much weather change expected, then I'll end up carrying more clothes, and even though I can sometimes jam everything into the GB, it gets so stuffed that it is a pain to use. I'd rather carry the Pendle and be able to get in and out of the GB bag without having to stop.
Pay attention to how your luggage affects your bike's handling. What takes a small effort to overcome on a 200km adds up over a 1200km to a lot of wasted energy and excess fatigue. You want the luggage you are carrying to "disappear" with respect to bike handling. That's why I like the Carradice bags--they don't affect handling the way that having a load cantilevered off the back of the bike does. As a thought experiment, imagine a 20-foot long rack cantilevered off your seatpost with your Arkel Tailrider at the end! Now imagine trying to climb a hill, standing up out of the saddle, with that swaying in the back. You want the load under and inside your center of gravity so that you don't notice it. Similarly, the higher up and further forward a handlebar bag that you have, the more it affects steering effort. For me, the convenience of a handlebar bag is so valuable, that it's worth the extra weight of the rack and decaleur that are required (along with low-trail geometry) to get the handlebar bag low and close to the steering axis so that it doesn't affect handling.
Nick