When I bought /installed the exhaust fan for my indoor spa room, I was recommended by the fan maker to size for 1 room air change per every 5 minutes (in CFMs). Larger rooms may need less. Multiply your room's length x width x height (the room's total cubic feet) and call up the manufacturer of whatever fan you are interested in, for proper sizing (we used Fantech) Our set-up works great, the fantech unit is an inline design, that is almost dead quiet.
Make sure you also include intakes for makeup air. In the summer we pull conditioned air out of the main house, instead of the hot humid air from outside.
Also remember you need a place to drain the tub, (or you could use a sump pump) and ability to easily fill/top off the tub. I have to add about an inch or so of water once every two weeks to ours. It is a PITA to have to drag a hose into the room from a distant water source.
Agree with the hottub.pool guy, with the cover on, there is almost no humidity gain to the room. But if you accidentally leave the cover off over night, with no fan running, the next morning the room will look like someone washed the walls down with a garden hose! (Been there/done that).
As for fan control, we use (again recommended by Fantech) a manual wall switch with an adjustable timed off setting. Turn the switch on to manually start exhaust fan. Turn the switch off, and the fan continues to run for another 1/2 hour. This way I can open the spa up, turn the fan on, and clear any "ozone off gassing" out of the room immediately. Since basements are already susceptible to humidity problems, you may want to go with a humidistat, and even check into also adding a small room dehumidifier.
Many folks recommend not using an ozonator in an indoor spa, if you do, make sure that you isolated that room from the rest of the house (we use a sliding patio door as the only access point).
As for the pro/cons of indoor/outdoor spas. We (here in SE Va.) have bugs in the summer, falling leaves in the fall, Nor-Easters in the winter (rain and wind, as oppose to snow), and all sorts of stuff flying around in the air in spring!
As my wife commented last night, an indoor spa, for us, is just easier.
My question is how the heck are you going to get a tub down in your basement?
