If you can wait, get the SP4. But you are betting for a end of the year release, which doesn't look like will happen mostly due to Intel delays. So probably Q1 next year. Definitely not January or February. These 2 months, people tend to not buy anything. So expect late February or March. So expect that much waiting.
The SP4 will have (that is pretty much expected), longer battery life, a bit faster CPU and faster GPU, due to the new Intel CPU (that is assuming that the rumors of that are correct). It will also be more polished device. MAYBE 2 USB ports (I expect full size USB 3.1 and the new TypeC USB). All depending on the space saved from the new Intel CPU, due to the integration of components.
Expecting a low end Nvidia or AMD GPU inside, while possible (a ~5W Nvidia GPU can be done, in fact that is what the Nvidia Tegra K1 ARM based processor, has that beats Intel best offering), I would highly doubt it. There is currently no market or interest dedicated GPUs. So while cool, and awesome for many, it's probably not a selling point for most, so it won't be in.
Thunderbolt is a possibility, but it's expensive to implement, unless Microsoft can get a deal like with Intel, like with Apple, to have it cheap. And really nothing uses Thunderbolt. Well you have, but nothing that can't be accomplished with USB 3.x. All I can think about is external graphics card, but:
1- the enclosures cost 1000$ for starting, you need a power supply and graphics card.
2- It's equivalent to PCI-E 4x. So not fast enough to have something worthwhile, especially that 1000$ starting price tag.
3- You need an external monitor, you can't have it feed back the image, back to the Surface Pro device
4- Driver support is not there.