Given that you have a spa and waterfall and assuming you use the same pump for all features, these features will pretty much dictate the size of the pump.

So the first set of questions I have are:

Are you happy with the current performance of the spa jets (e.g. strong, weak, etc)?

Are you happy with the current performance of the water fall (e.g. look, sound)?

Do you know the model # of the pump? If not, is there a service factor (SF) listed on the pump motor label?

Are you planning to add solar in the future?

What do you pay for electricity ($/kwh)?

A variable speed would give you a lot of flexibility so there would not be any guess work when sizing. Also, for a given flow rate (RPM), it will use less energy than a two speed at the same flow rate.

But in general, both a two speed and VS pump will pay for themselves over the life of the pump, it is just a matter of how long it takes.