For 35', you could use 16 ga and never hear the difference. For 6', whatever you use will be fine- for a run that short, it won't matter at all. If you plan to use a really high power amp and crank it all day, every day, get some 16-4 and be done with it. You can double it to the mains if you want and it will be plenty for the surrounds. If you run a piece to each speaker, it will keep you from needing to run more if you decide to go with dipole or bipople speakers at some point.