I prefer the variable speed to the two speed. The main reason is that the low speed on a two speed is two slow for many applications and the high speed is always 3450RPMs. With the variable speed you can customize all speeds to what you want and need. On some of my simple variable speed installations there are only 2 speeds programed but they are the speeds that work best for the particular job. For example I have one on an old pool that runs at 1700 RPMs most of the time for circulation and ramps up to 2800 RPMs for 3 hours each day to run the suction side pool vac. This pool has no spa, no solar, and no heater. The client saves over $100.00 per month compared with the single speed 1 1/2 Hp that used to power the pool.
At my facility we had 3 challenger pumps (3 hp), one on our 18,000 gal therapy pool, and 2 on our 1,500 hot tub, with one running the filter & heat exchanger, and the other running the air jets.
Under our previous maintenance director, we were replacing them all the time (not his fault - the contractor who plumbed them in had elbows directly into and out of the pumps).
We have since replaced 2 of the 3 with the intelliflo pumps, and they cost us about $900 apiece, roughly the same as what was quoted us to go with the challengers. When installing them, we corrected the plumbing to meet the requirements in the manual. Going from 3450 rpm to 3110 rpm in our 18000 gallon pool brought us from 3,000W to about 2,000W 24/7 for an identical flow rate, using the pump speed as the throttle instead of the valve immediately thereafter.
Our second replacement was the whirlpool jet pump, and we went from 3,000W at 3450 RPM to 1,000W at 2350 RPM, with the same flowrate as the old challenger pump. At night it runs at 750 RPM (we have no timer - it would've cost us almost $3,000 for the local electrician - new pump was 1/3 the cost!), at about 100W. We figured running it slow overnight at 100W would help keep things from growing in the pipes vs. stopping flow altogether.
All in all, we are so far very happy with the intelliflo pumps.
That sounds like a great testimonial for VS pumps.