I am not sure if one is more rigorous than the other, but keep in mind that D1 has a longer season than the other divisions, which in itself makes it a bit more difficult to remain as rigourous as you started out. I would imagine that all programs that meet top 15-25 criteria in their divisions have outstanding programs because the expectations are higher, that would be on the field as well as in the classroom (to remain eligible).
I do believe that the reason some people feel the top D1 programs may be more rigourous is because the talent level you have to compete with and against prevents you from slacking off, if you want to keep your position. There is also a huge amount of pressure on coaches, players that are on teams that make playoffs and often expected to go to the CMWS or the WS in their division.
Having a player that attended a highly competitive top d1 program in the ACC, I can agree with jemaz that it was almost like having a full time job, plus 18 hours of class each week and study.