Which rMBP has a 4GHz CPU or Radeon 580 GPU? To me $2899 for a 3.1 speed CPU is not a good value when I can have 4.1 CPU plus a 5K screen for basically the same price.
You asked which one we would rather have.... not which gave best performance...
My own opinion, as someone who has most of these options already and not wanting to argue, the screen on the iPad Pro is too small and too shiny for serious use. It will do your head in. Hell I hated it for reading email never mind getting colours and exposure right. A sunny day will render it useless and all the faff of transferring files slowly will mean in a month it will not get used. Once you put a case on it, it isnt that much smaller than a rMBP.
The editing apps are close but not quite there with their desktop counterparts causing a difference in flow for you. Once you stop doing the editing on it, then it is a comedy big Kindle reader that you cannot use during take off and landing as the air crew think it is a potential projectile. Something I learned the hard way. I thought it would be great for editing documents in transit but it just feels stupid big to me and I have gone back to my 8" Samsung which works great for what I actually use it for. In fairness my daughter loves watching PowerRangers on it. Of course YMMV.
I would value a consistent editing experience in the field and back home with fast file transfers if I can get it in exchange for flat out performance. Especially when the performance difference isnt going to be great enough to really impact your work. IMHO.
In your question, you seem to want to be told buy the iMac... if you want the iMac go for it. They are great machines or.... buy both...