I would go with the buffing machine that spins @3,425 and here's why:
One facet of buffing that doesn't immediately make itself known is the fact that even though on paper the larger wheels seem like the be-all-end-all solution to building up enough heat to make the compounds work effectively, the catch-22 of this is that sooner or later you're going to encounter a part that has a tricky set of angles that the larger circumferenced buff simply will not fit into. Sure, most buffs will mush into tighter spaces if enough pressure is applied, but that begs the question of whether your buffing motor puts out enough torque to keep the wheel moving while you do this.
The higher spinning motor will give you versatility. You won't
always be able to get away with a 14" wheel to get the job done, so in the instances where you have to switch to a smaller wheel you'll still be cranking enough RPM's to allow a decent SFPM rate. Also, a fact of life is that buffing wheels deteriorate, so as they shrink and your SPFM's decrease you'll want to be able to maintain the highest rate of surface speed possible to squeeze the maximum usable service life out of the buff. I don't imagine a 4" or even a 6" wheel would be worth a damn at 1,725rpms. (Unless you were buffing plastic.)
Here is a water pump from an LT1 engine that I did recently that sort've demonstrates this point. Notice on the right hand side of the pump underneath the water inlet, how there is a "valley"? Squeezing a wheel down in there (and not getting it snagged and shooting the pump across the room) was a challenge. NO WAY I could've done that section if I were trying to use a 14" wheel. It is the same down on the bottom around the areas that have the circular projections. Just something for consideration:
