I haven't tried the Clarett - but if I had a PC with Thunderbolt I would have gone for the Clarett (much more expensive than the Scarlett though). I have heard nothing but praise for the Clarett range.
The 2nd gen Scarlett comes at the same price range as the first gen - really cool move.
I've been through a dozen USB interfaces the last few months. Started with a 6i6 Scarlett and wanted something more "high end"... so went through a few of those fancy USB interfaces - and in the end returned to the Scarlett range (18i8 1st gen currently)... IMO, they offer the best package at a great price - hard to beat.
Every other high end interface had some annoyance I could not live with (eg no power switch, no master output, no SPdif, not enough output juice, crappy drivers, you could not bypass the preamps on some when sending your KPA through it...). The Scarlett range has done everything right!
Thank god for the Thomann 30-day-money back thingy...