It's really close, IMO.
Nadal had the better competition, but Fed hsd to beat a top 5 player in every GS semi in 2005, which is why he couldn't win more than 2 slams. Nadal also lost to too many players that were not Big 3, so it isn't the fault of the competition that he lost more.
Nadal was supreme on clay, while Fed was much better on HC. On grass, they were about equal.
Overall, it took a higher level to beat Fed than it took to bwat Nadal, despite the impressive 32 match winning streak Nadal had in the middle of the year.
As a season though, 2008 > 2005 at the top, but 2005 had more exciting matches overall than 2008.