Yeah they're separate, although there's definitely an affiliation there. And IMHO earlier versions of WKO+ didn't really offer anything over TP Premium for most individual users, but WKO+ 4 has a completely new set of metrics not available on any other platform (including TP), so it's worth checking out depending on how serious you are. IMHO the new PD Curve-based metrcis and optimized intervals are especially useful for focusing on above-threshold work.
Okay, that makes sense. One thing I've noted in researching and testing the various platforms is that if you combine two it would probably make sense to combine ones that uses the same model. I have found for example, that Strava and SportTracks uses the Banister model. TrainingPeaks and Golden Cheetah use the TSB model.
The differences being
highlighted here
So I suppose depending on the model you subscribe to, may dictate which tool you opt for. I don't know which, if any tools, use the Busso Model.