Depends on what you're gonna be using the Surface Go for!
Personally, I'd upgrade, as you will see a performance increase of some kind compared to what you get with the Intel Atom chip on the Surface 3. If you're gonna be using Photoshop or doing video editing, then I'd go for the Surface Pro, but for casual web browsing, emails and some very light gaming, I think the Surface Go should be the upgrade to go for!
I've pre-ordered the Surface Go 64GB / 4GB configuration myself, upgrading from a Lenovo ideapad with an Intel Celeron chip, and will also replace my Surface 2 RT (I know, old right?!).