Welcome Jon. If you wish to, like many of us have done so, you can already install GDR3 (also called 'Update 3') onto your phone right now. The only real downside to this (for some on contract for example), is that Microsoft say that as this update is not specifically issued from your carrier, it may void your carriers warranty. Please see
this thread for more details on this.
If we don't have GDR3 on the 520 on AT&T by now and they're releasing another larger version update at the beginning of next year I'm guessing we really are not going to get either.
You will get the updates, but there is no specific time when you will receive them. The date when Microsoft finishes an update, is not the date you will receive it OTA to your phone. It's up to the carriers to release that update after testing it. Thankfully with GDR3, Microsoft enabled us to manually install it if we wish to, as I mentioned above. Your first update will be GDR3/Nokia Black Update, then (likely a few months later), you should receive the large 8.1 update.
My inclination due to uncertainty is to rev up the old iphone (2 years old and still beyond my WP8) and send that stupid Surface back when UPS brings it to my door tomorrow. I really love the start screen on my WP but it's not worth not being able to close an app or stop my screen from rotating continuously either...
I'm not sure this should put you off owning a Surface, which is lovely device. The Surface is a completely different product, that can do a lot more than a phone can. I'd at least give the Surface a try