Sixt are actually based in Germany. I wouldn't call them "budget" here in Europe -- that would be the local no-name outfits (most of which are quite good if you don't mind a longer schlep from the terminal and old cars). Interesting that Sixt has outlets in the US -- maybe that's to draw fidelity program users.
I've used Sixt in a couple of countries and found that their prices are often rather high compared with, say, Budget, although Spain seems to be their sweet spot.