I think it is price and styling between those two. The FLO is 1mm wider and about 80g lighter. But it is $275 more costly. If I had a FLO front, I would get the FLO rear disc. If I had any other front, I would get the Premier.

I still think the FLO aluminum + carbon is perhaps the best deal out there for a new disc. At $674 it's a few hundred less than the FLO carbon clincher, slightly less than the Premier Disc, FLO's own data shows it's slightly faster than their carbon clincher version, it reportedly weighs slightly less than the Premier (claimed 1344g vs 1390g), and you get the better braking of an aluminum rim.

I really love everything Dan's doing with Premier, and I can't wait to see how the road bike turns out, but the FLO alu+carbon is simply a great deal.

Braking surface
Flo-alloy
Premier-carbon