1) Factory Unlocked M8 : I am unsure of the Root status and which carrier tree this would fall under and also AT&T LTE Support.
2) Verizon M8: I read that phone doesn't support AT&T LTE and only HSPA+? I also read that this phone will be entered into verizon's database so it would be more valuable resell down the road as you could use it with verizon or at&t? ( i have never been a verizon customer so don't know the benefit or process for bringing your own phone)
Thanks for your feedback!