Having never heard of the TATR, and knowing the BHX organiser, I'm riding the BHX grand depart in September. I'm hopeful that it'll be a good ride, photos and reviews make it sound like hard work but I'm up for a challenge.
The one thing I did notice on the TATR site was "may be a bushwhack" - not a fan of that!
I'm leaning towards the BHX. If you look at mileage and finishing times: BHX = ~460 miles, done in 4+ days, vs. TATR = ~550 miles, done in 8+ days.
It's clear that the TATR is either way harder, or you are off your bike a lot more, which I believe is the case. At any rate, I'm only going to have 10 days available, and I don't want to spend half, or more, of it HAB-ing. I'd rather finish up the BHX and have some time to explore the area and maybe hit some of the highlight singletracks out there.