A larger motor will pull:
a) a wider blade through stock
b) a blade with more teeth -- thus a cleaner cut, depending on other
factors
c) will cut through stock not completely dry.

I'm moving to a national forest in 3 weeks. When I ordered my General
350 TS 6 weeks ago, the tool dealer tried to talk me out of a bigger
motor. He was trying to save me money. When I told him I'd be
harvesting timber from time to time, he said, "OK. Get a 5hp. It's only
$125 more".

If any of these factors are important to you, get a bigger motor.

GC in L.A.