I have never used Sound King wire before but I have read many good reviews and Partsexpress.com is a very good company. Any standard speaker wire like Sound King, Radio Shack brand, Carol (Home Depot), and others will be fine.
12 Ga. will be fine. No benefit will be gained by going with 10ga. Hell, with 15ft runs, no benefit will be gained with 12ga over say 16ga. However, 12ga looks cooler.
Shinerman