I’ve been pondering this question after recently building up an FF29 hardtail and comparing it with my Trance full suss. I rode the FF29 for five and a half hours on Saturday, which helped me crystallize a few thoughts on the relative merits of hardtails.
First, maxtourque is right, frame material is completely irrelevant. The difference between the stiffest and most flexible frames out there is only the same as a few psi more or less in your tyres. Forget about it.
If you want to make a hardtail more comfortable for all day riding then look at: bigger wheels, to roll over bumps more easily; a larger volume tyre, for some extra cushioning; a comfy saddle; a flexible seatpost and the longest chainstays you can find, so any bumps are happening further behind you and cause less deflection at the saddle.
Personally, I think a hardtail makes a good all day bike so long as the riding involves plenty of road and smooth trails where the extra efficiency saves you a decent amount of energy compared with a full suss. It’s also a good option if you are going to do plenty of hike-a-bike as it should be lighter to carry. However, if it’s going to be all day riding on bumpy trails then give me a short travel full suss any day. With a full suss you can sit on some bits where you’d need to stand on the HT and when you are going down bumpy stuff you don’t have to work as hard to absorb the bumps with your legs. All of which saves you a lot of energy, which is the key to an all day bike as far as I’m concerned.
Well, that’s my thoughts anyway.