They're so completely different that it depends what you want. You should probably think of the surface more as a laptop, minus the keyboard because that's how it behaves. It's heavier than the iPad and has a shorter battery life however it is more powerful and will run any legacy windows program along with the apps from the windows store. The iPad is lighter, has a longer battery life and a much larger selection of apps compared to the Microsoft store. Whilst I have a surface pro 2, there's a family iPad that I use too so trying to be non biased.
I don't get the point about the iPad having more apps, considering the Surface Pro runs full Windows. As mentioned above, think of the Surface as essentially a touchscreen laptop minus the keyboard, and can run fully fledged PC programs like a traditional desktop would. However it's bulkier, heavier, has inferior battery life and doesn't have a display as fantastic as the retina panel Apple use. iPads also have a better library of touch oriented apps.
If you want something productivity oriented, the Surface Pro is flat out better. If you want a lifestyle product, the iPad is likely going to be the more enjoyable choice.