Thats a tough one 6 feet is like 180 cm? Wow, thats a toss up. I dont know much about either ski, other than demoing them, but i would assume if you rode mostly park, the SPK would be the way to go.
I found the foil to be a little softer then the spk, and more turney and quick. I really liked them, but, the felt a little bulky.
The spk's were awesome. Super light, great switch, super fun.
Since your 170lbs, i would go with the SPK's if I were you.