If the only options to consider are the Surface Pro 4 and the iPad Pro, stick to the Surface Pro. The iPad has always been a cool toy that you hand over to your kids when you need to get work done. The Surface Pro *is* the work machine *and* the play machine.
The Lenovo Yoga is better than both for anything other than traveling, in which case the Surface Pro has the edge (smaller screen, thinner, lighter weight) but it is platformed on older chip technology. That won't matter much at the time of purchase, but it might in 2 years (Surface Pro 4 is Intel Skylake based and the Yoga is Intel i7 based) I know that the Yogas are supposed to move over to Skylake-based chipsets, but I'm not sure if they've made that upgrade yet. (I haven't been in the market for a machine in a while, so I'm a touch behind on current details)
My advice is that unless you are purchasing something that is only to be used as a toy, discard the iPad from consideration. Go with either the Yoga or the Surface Pro.
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