If you buy a Powertap wheel, and don't have a head unit already (an Edge, etc), the Joule means you don't have to go out and drop $300 right away.

It might do a better job of talking to your PT than the Edge. I have no idea, I've never used a PT. I've heard people say that about SRM, though; their power meter apparently can broadcast 12x per second and ANT+ only does 4x so their cranks "dumb down" when you use them with a Garmin. I've never used an SRM, either, but that may be useful if you're doing aerodynamic testing (which is much more demanding than training is as far as accuracy goes).

If you're using your power meter for training purposes, what matters is that it's consistent, and that's going to be the case no matter what head unit you use, so stick with your Edge.