I've played 109k tracked hands @10nl full ring on Stars and 121k @6-max 10nl on Stars. Rake rates for me are:

FR: 5.6bb/100
6m: 7.2bb/100

but I play around 30% more hands at 6-max so hardly surprising. I expect the rake is more of an issue for people that play for a living and at levels where their edge is smaller, as a couple of bb/100 multi-tabling at 100nl+ can make a big difference.

Imo, it's much easier to transfer from 6m to FR than the other way around as there are many more marginal situations at 6max and the game is much more aggro. I enjoy 6max more for this reason but I also play it now because I'll improve far quicker playing shorthanded and there are sufficient amounts of fish at both FR and 6m. The variance at 6m can be a bitch however and require greater resilience and bankroll to handle, so ultimately I'd recommend the format that suits your personality the most.