The Retina MacBook is more powerful then the iPad
rMB can:
Run multiple operating systems, via bootcamp, i.e., Windows, Linux, etc.
Run operating systems virtualized, i.e., Vmware.
Run desktop apps like Photoshop, Lightroom, Creative Cloud
Multitask beyond splitting the screen in two.
Use a multitude of pointing devices, including to digitizers, mice, trackpads.
More ram
More storage
Accessible files system.
The MBP has more benefits, including
dGPU, larger display, more ports
As for waiting or buying, if you're want the MacBook, it was just updated so buy that now, for the MBP, I'd wait if you can, though I think the current model is quite powerful and you'll not regret that.