Go for the 51mm offset. Much better. The theory is that if you increase wheel diameter the Trail is increased (well, either increased or decreased I forget which) which has the effect of making the steering feel slower. If you increase offset you counteract this and it speeds up the steering. The downside is that it slightly increases wheelbase, but the feeling of quicker steering easily overcomes that. I also got more of a flip flop effect from the front end – i.e. if you turn the bars you get a self-centring feel, but with the greater offset there comes a point where the bars suddenly switch from a self-centring bias, to a hard over bias. Not a problem, a bit strange to begin with but you simply get used to it.
For me the handling of the bike felt transformed going from 46 to 51mm offset.
I think Fox Forks are already 51mm offset anyway with RockShox defaulting to 46mm.