Yes, go with the slower of the TWO. Keep in mind that the buffer is commonly called the most dangerous machine in the shop. I have seen pictures from full time professional knife makers with a knife sticking out of the ceiling. The wheel caught the knife and threw it.

They were using loose muslin wheels and you have to use them for a mirror finish.
Also keep in mind that the size of the wheel determines the surface area that the knife sees. So, really you can change "wheel speed" by changing the size of the wheel. K&G has a chart on the top of page 20. Actually the RPM is measured by the working area that the piece being buffed sees.
1725 RPM wheel with 4 inch wheel is 1800 RPM.
1725 RPM wheel with 10 inch wheel is 5475 RPM. So, really you can change the speed by having a 10 inch buffer and using different wheels.
At 3450 RPM these numbers more than double and shred that loose muslin pretty quickly and is more prone to throwing a knife. Greater speed - Greater velocity!