I think if you can do it, the 6.5 month rotation should produce some really great results, but for me, it's more of a personality thing, I just don't see myself doing the same mesocycle for 2 months straight. 4 weeks is about just right for me before mentally needing to move on to something else.
I even recall Cathe posting that the 6.5 month is her preferred recommendation to get the absolute most out of the program, but they understood many people are not able/willing to commit to such a long rotation and will still get fantastic results with the 3.5, I guess mainly psychologically 6.5 months can seem overwhelming.