I'd recommend a desktop system with Intel 4-6 cores with 8-12 threads. Outfit it with 8GB+ of high speed RAM, and fast drives for audio work: A combination of HDD and SSD.
 
For mobile work, a laptop with a Intel Core i7, 8GB+ of RAM, and internal SSD - 512GB minimum.
 
I'd go with W8Pro x64. No problems to report with Microsoft's new OS.

i have been using a windows laptop for a while because of portability. im interested in getting ubuntu studio working with my fireface ucx in class compliant mode. so far, i dont have it working. i think it will work though. then, i take my removBle drive anywhere with the firface, use any nearby handy pc.. think my work laptop.. and make a field recording. im hoping to buy a serious desktop pc for home use in november.

shawn, I'm with you.  I wouldn't go back to a desktop.  

For pros that live in a studio I can totally see the reasons for a desktop. 
 
For a home studio guy, like myself, the freedom of being able to attach to the gear in my studio setup just like a desktop, or to be able to detach and sit anywhere else to edit and/or record stuff is just too convenient.
 
I wouldn't want to give up the flexibility of a laptop now that I have it since I can do absolutely everything on it.  I get more done with it than a desktop.

The great thing about Sonar is you can have both :-)