You're probably the only one who thinks that. 81 on clay is far more impressive. a) It's a lot more - 81 matches is a lot. b) That's over Masters. 500s, slams - lots of events, lots of players. Fed's are Halle and Wimbledon - winning best of 5 matches on grass at his peak wasn't that hard for him and an upset hard to pull off. That leaves winning Halle 5 years in a row - Nadal's won MC 8 years in a row - a best of 3 tournament; won Barcelona 5 yrs in a row too...
Both are great but the 81 on clay is more impressive.
I agree with you that both are great stats BUT it's much easier to get upset on grass than on clay. A big server who can hit big has a much better chance of making a huge upset on grass than on clay. Think about the first round of Roland Garros last year where Isner took Nadal to 5. Had that been grass who knows... the upset would have been more likely. Upsets on clay is really not all that likely. Even Federer himself kept making the FO final over and over and over and losing to the greatest clay court player ever, Nadal himself.
If Nadal was not so damn amazing on clay we would most likely be looking at Federer with about 6 French Opens right now. Just putting things in perspective. Without Federer, Nadal still would not have any where close to 6 Wimby titles.