We have to wait until both careers are over. Federer got his game together later than Nadal did, but that does not make Nadal's achievement any greater. Seven slams is seven slams.
Nadal's game will almost certainly drop off earlier than Federer's (looks like it's already happening) so the losses could even out in the end. If Nadal loses the next 4 or 5 Roland Garros tournaments (very possible) then we can say that the dominance at each event was equal at the end of the day. Also Nadal didn't play in 2004 event despite being on the pro tour already, and therefore avoided an early RG loss.
All that aside, Wimbledon is the biggest tournament in tennis. The one tennis tournament that everybody - including Nadal - wants the most. So it stands to reason that it is where the competition is at it's most fierce.