Half the price? The international unlocked Lumia 830 is going for $300 these days. The 640XL is supposedly going to go for $250, and we all know the unlocked prices will be inflated to start. There won't be a big price difference, let alone half.
As for which to choose, I think it simply comes down to how big of a phone you want and how big of a role aesthetics play for you. Although the 640XL is nice, it's no where near as beautiful as the Lumia 830 in my opinion.
As for less subjective differences, I doubt the 640XL's camera will be better. I didn't hear anything about "PureView" tech. And the Lumia 830 has built in Optical Image Stabilization. If I'm not mistaken, the 640XL will not have that.
The 830 also has built-in wireless charging. If you have any wireless charging accessories like I do, that's a big plus.
In the end, they're both mid-range devices, but the 830 is a little closer to flagship status with the added features mentioned, in addition to a more premium build.