I am naive. Why would I need/want waas?
If you intend to use the unit to fly IFR, you'll need WAAS. This includes en-route and approaches. You can use a non-WAAS unit for situational awareness, but as your sole GPS RNAV instrument, it would have to have a current database and be WAAS capable.
WAAS provides an incredibly accurate and enhanced positional augmentation as compared to non-WAAS GPS navigation.