To me it looks like the iPad Pro is predominantly aimed at the design professional, whereas the Surface Pro is aimed at the general professional, ie microsoft suite of apps.
We've not had any proper specs about the processing power of the iPad Pro, but on paper, it looks like the GPU is geared up to allow for pretty impressive performance which will assist designers. You certainly pay that bit extra for the 128Gb storage compared to the basic 32Gb, but at least there is the option. The keyboards for both iPad Pro and Surface Pro are expensive, so 3rd party versions for both will help with costs. And the iPad Pro had a slightly larger screen and is slightly thinner and quite a bit lighter than the i5 version of the Surface.
It will be interesting to see where this goes though.