I'm not familiar with those particular implimentations, but I did do a variable speed mod to my 8x14 and love it!
The slowest speed I had with belt-changes was like 650 though, so it made threading even in low gear a hairy experience.
I did notice on my last project (In the thread "What the chuck?") that I was having some torque problems on mine. I'm not sure I wouldn't have had that problem with the stock motor as well, so it may not be an issue overall.
Really, the best of both worlds would be a couple of belt ranges (to get the mechanical advantage) and a variable speed motor.
I'm considering re-doing my setup to work that way. In general, the variable speed is very nice. Having a really low "threading" range with lot of torque and a "normal" range for turning would be just about perfect.
Generally, I'd think it would be easier/cheaper to upgrade an electronic speed control setup to have a handfull of mechanical ranges than vice-versa.