Depends upon electrical loads and what you wish to do. For a VFR airplane, the vacuum pad works great. The newer B&C alternators actually put out 40 amps and seem to come online around 1200 RPM's. I have verified it will carry the RV-10 Full up load load through a descent, shutdown, startup and climbout, and have the battery fully charged about 15 minutes into the next leg.
If you are planning a full up IFR capable airplane, then a belt-driven one with a standby gear-driven one is usually a best practice.
Vic