I prefer Asus and Gigabyte motherboards, in general. Asrock makes pretty decent stuff as well. In fact, I use an Asrock in my home server that runs 24/7 and Folds nonstop as well and I have NEVER had an issue with it (or with any previous Asrock products).
Honestly, you can't make a bad choice between the 2 of them. Flip a coin and have fun!