Are the Sierra-1's in your budget? I might be selling my Sierra-1 center channel soon, and that would save you a ton of $$. You could also do a CMT-340 left & right, then use a Sierra-1 center.
The Fusion-10's would have more Spl, greater dynamics, and most likely have more mid-bass than the 340's, but the 340's would probably have more detail and have better sound staging. If it were me, I would go with the Fusion 10's over the 340's, but I would take the Sierra-1's over the Fusion 10's.
If you went with a pair of Sierra-1's for the left & right, then a used Sierra-1 center channel, then use the 170's as surrounds or wait on purchasing surrounds until you can save up more cash so as to have better LCR's, that would be the best option, IMHO!