The iPad has potential, but Lightroom for the iPad is crap. You can connect an SD card with RAW files on it to the iPad... but Lightroom can't read them. Won't even show you a preview (even though Apple's built-in app can). Not good.
The iPad Pro is a way better device on paper than the Surface. But being able to run every Windows program ever is better than being able to maybe get a stripped-down version of the software you're used to.
I imagine sales numbers between iPad Pro and Surface Pro 4 are orders of magnitude in favor of the iPad for two reasons. One, the Surface Pro 4 competes with every Windows laptop ever -- there are many cheaper ways to get all your favorite Windows programs "on the go". Two, I bet a lot of people bought the iPad Pro imagining things like Lightroom would be useful, discovered they aren't, and put the iPad in the drawer two weeks after buying it. I loved my iPad for the first two weeks after I bought it. Now I don't know where it is. Dead in a drawer somewhere, no doubt.
The Surface series, meanwhile, creates a new platform; new apps (or at least new major versions of apps) have to be written to truly take advantage of it.