while positioned as direct competitors, both devices are pretty different.
iPad is a blown up phone, and that works.
The Surface RT and 2 with their "Windows RT" software is a shrunken computer. And that works too.
In a nutshell, I would say the iPad is better for games (iOS is the de facto game mobile platform for the most part from what I see) and the RT 2 won't be able to take advantage of PC games like Starcraft. (not talking about other Windows tablets, like the Surface Pro 2, Dell Venue 8 Pro, which can.)
The Surface is better suited for advanced work, with Office onboard, first party keyboard solution and a USB port. It can handle lots of USB peripherals and deal with flash drives etc. unlike the iPad. Windows RT is more powerful than iOS with split-screen multitasking among other features (like the desktop) however it doesn't necessarily mean its better. For me, it would be, but other people might not care as much for those. (I won't mention x86 compatibility as both can't do it)
But it doesn't mean you can't use either one for everything. Windows has games in the Store, Halo: Spartan Assault is quite good, theres a GBA emulator and the iPad doesn't necessarily suck for productivity, there are keyboard solutions up the wazoo and Office for iPad is a nice option for 365 subscribers. (probably businesses)