Re: Nadal leads Federer 3-1 outdoor hardcourts, while Federer leads 3-0 indoor
I think there is something like 30 total points that seperates the two players in the 22 matches they've played, which is pretty incredible. If you take away Rafa's RG 08 win, its basically equivalent.
The point is, almost nothing seperates the two, and hence tiny little variations or surface changes can swing the match one way or the other. A point here, a point there, etc.
From the matchup that i've seen, my general observations is that the biggest factor in deciding the match is how well the surface responds to Rafa's spin (both serve and topspin). The next most important factor is speed. Fed prefers faster surfaces where Rafa can't run as many balls down, and consequently he doesn't have to go for as much and makes fewer unforced.
The final one is important, but we haven't seen it too much of a factor. Namely weather. Generally speaking, i'd imagine Fed would prefer as little of it as possible, and Rafa as much as possible (wind, hail, blazing sun, tornado, whatever).
I'd say that if the outdoor hardcourt was very fast and didn't respond well to Rafa's spin (say the USO), i'd pick Federer (at least in his prime) over Rafa as he is a more gifted and natural hardcourt player in general. However if its slow and there is bad weather, i'd be inclined to give Rafa the edge.