Warning! Answer from a not-yet-pool-owner!

Ken and Laura - How much more does the 2-speed pump cost than the single speed pump?

I assume that, if you chose the single speed pump, you would size it to perform the same as the 2-speed pump would at low speed.

Even without a spa, I would say that having the option of high speed operation would be desirable if you ever mess up the chemistry of your pool and want to circulate a lot of water quickly.