I sat in both while at Oshkosh a few years ago, once I sat in the fastback there was no other way to go for me.
The extra room was great(I'm 6ft5in), it was very noticeable to me.
Moving the roll bar behind my head has two advantages: better visibility and a nice handle to grab for getting up onto the wing/into the back seat.
The tip over also makes it easier to get into the back as the canopy is out of way instead of partially out of the way like the sliders.
I've not flown mine yet(finishing up the canopy now) but I hear it's less drafty than the standard.
Higher likelihood of ejectability of the canopy
FWD instrument bay/panel access
I like the fastback appearance
Fast is in the name so it must go faster