1) Standard lathe-type turning is far faster and more efficient than a rotating tool on a rotary axis, and will often give a better finish.
2) That's a nearly impossible question to answer, for many reasons.
Changes in diameter will greatly affect both lathe and spindle RPM.
Depending on the shape being cut, you may have situations where a LOT of material is being removed in some areas, and very little in others.
Amount of material being removed will come into play.
The type of tool you use may have a lot to do with the shape of your parts. A large ball nose or core box type bit may be a better choice.
Generally, with normal CNC routing, you want to feed as fast as possible, with the lowest spindle RPM possible, and still get an acceptable finish. Unfortunately, only trial and error testing will give you the answers you're looking for.