@Dashrender said:
Definitely upgrade.
I agree with the 635 though - I have one, I really like it even though I'm running beta Windows 10 on it and it has issues.. they are beta issues I live with.
Now I'm waiting on the 640 XL
I have the Lumia 1520. Running 8.1. I like it pretty well. I tried the 920 a couple years ago one 8 and hated it because of some of the issues and lack of apps back then. Now all the appps expect my SSL VPN are there. I tried windows 10 on it. I can't do it. I use windows 10 one my laptop and can live with temprory usability issues but I can't live with the design issues of text being outside of bounding boxes etc that windows 10 preview had on the 1520.