Well, first, they are different types of devices.
Surface 3 is a FULL Windows 8.1 device with a free upgrade to Windows 10.
The Nokia 2520 is a Windows RT 8.1 device and right now it looks like we will not get Windows 10 although we might get SOME of the functionality of Windows 10 in the form of Windows Updates.
The battery on the Surface 3 I suspect will not be as good as on the 2520 but it won't be too far off.
The Surface 3 will start at $500.00 for the base model. A bit high in my opinion.
The Surface 3 is a very nice device and I am not trying to take anything away from it. The processor is going to be better than the processor on the 2520 with it's burst speed of 2.3Ghz. It will also support charging from a micro-usb port via any USB connection including from like a PC or laptop (like you would charge a cell phone).
This is very nice for a Windows tablet.
For most things, I would still prefer my 2520. I don't have the virus/spyware issue to worry about and I believe the 2520 is still going to have a smaller foot print and a brighter screen with a better battery life.