I have a buddy who just bought a Hammond XK3c system, and he already has a 147 Leslie at home.
In his large rec room, with the XK3c running (in stereo) through a Stagepas 500, and then through the Leslie 147, there is a difference, but it is slight.
The XK3c (and Chas's Nord C1) have incredibly accurate Leslie simulators...of course they don't actually move air and have the Doppler effect (as Russ points out), but for nearly all situations, the simulator would work just fine.
A mic'd Leslie (through a PA) generally only provides the Doppler effect to the player.
Another consideration is recording...the sim is so much easier to record than setting up mics for a Leslie tone cabinet, and on a recording, it's almost impossible to tell one from the other.
Have you tried the Ventilator?
Ian
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Yamaha Tyros4, Yamaha MS-60S Powered Monitors(2), Yamaha CS-01, Yamaha TQ-5, Yamaha PSR-S775.