The things the 640xl has going for it, over the 650:
- better camera
- better battery life
- better processor
- bigger screen
The things I like in the 650 compared to the 640xl:
- windows 10 mobile support to last for a while
- better design (just the looks of it)
- more internal memory
If you need to use the WDRT with the 640xl you'll go down to 8.1 and will have to upgrade all the way back to 10, but with the 650 you'll stay on 10. This imo is a big deal.
Ultimately I would stick with the 650, because it shipped with w10 and you're sure it won't be dropped from the supported list anytime soon. The only reason I would go for the 640xl if I was in your place; better camera and bigger screen. Unless you'll be using the phone for watching movies the whole time, the bigger screen shouldn't be an issue lol