Steve's comment in October seems intuitively correct, but is not quite right with respect to VFD lathes, and we see this frequently in the forums. It is torque that rotates the spindle against a cutting load, NOT power. Power is the product of torque and rpm. If the torque is held constant and the rpm is reduced, then of course the power is reduced, but not the ability of that constant torque to rotate the spindle against a cutting load. Beyond that, the VFD unit provides the manufacturers with considerable latitude in "tuning" the torque curve of the motor to fit the application. For wood lathe use this can, and does provide very high torque at low motor speeds, even though the power produced by the motor is much less than rated hp.
I think that we sometimes place too much emphasis on which is more important since they are directly related, but with a variable speed drive it can be confusing, if not misleading because of the way that things are advertised. Just to get technical for a bit, in order to do work (cutting wood in this case), power is required. Torque does not necessarily mean that any work is being done since the motor is generating torque even when it is stalled. Three phase motors designed for inverter duty are generally capable of producing full running torque at base speed and slower -- all the way down to zero speed in the case of a true vector feedback system. Consider, however, that a one horsepower motor can only produce three pound-feet of torque at base speed and it becomes easier to see why torque alone isn't going anywhere. Power, the product of torque and speed, tells us how much work can be done. With a variable frequency drive, this also means that below base speed (base speed is the 60 Hz speed), available mechanical power output is reduced proportionally with reduced speed. Lathes with variable frequency drives are going to wimp out at some point as the speed is lowered. Lathe manufacturers deal with this problem by using stepped pulleys having at least two speed ranges. The purpose of doing this is to keep the motor speed up high enough that adequate
power to the load can be produced (remember that available
torque is constant). Stepped pulleys are also important to minimize overspeeding the motor. This is straying off the topic a bit, but running the motor at very at high speeds is undesirable for a variety of reasons including reduced torque and power.
Torque is important when considering loads (such as loads on gear teeth in a drive train), but it is power that actually does the work. Torque can exist in a static situation where nothing is moving and no work is being done.
I think that while VFD drives are great, when someone is shopping for a lathe, it is important to recognize that everything has a down side that balances the good news. With stepped pulley drives, the advantages are lower cost and more power while the disadvantage is limited speed control. With variable speed drive, the disadvantages are cost and reduced power at slow speeds. The better lathes compensate for this with multiple speed ranges (we're back to stepped pulleys again) and larger motors (more money). So, it is important also when comparing lathes to recognize that higher horsepower motors on VFD lathes does not automatically translate to more powerful -- more likely it means getting back on an even keel with fixed speed lathes.