Well you definitely use your phone more than I do.. Selfies - yeah just kill me. I've taken I think 10 in my life.
Sure the processor is faster, but if you can't tell any difference in app performance who cares? and it's probably just eating your battery faster.
Clearly the camera is better with more options, assuming those things are important to you. I didn't use to care about it.. but after having the crappy camera in the 635, yeah, I care at least a little.. and the 640XL is much better than the 635.
Screen size is a personal thing, not really advantageous or not.
How long does your battery last?
I'm pretty sure my 640XL is the LTE, internet is pretty damned fast on it - to the point that I almost never join an open hotspot because they are WAY slower than my data plan.