Performance wise? Any AOSP or GPE ROM had felt the same to me. Very fluid performance with no lag to speak of.
Battery life? Haven't used both enough to get a grasp but from I've seen this leans more towards gpe based software.
Stability? Crushing victory for GPE here. Their Roms contain all the proprietary stuff from HTC so naturally just about everything hardware wise is going to function just fine. All this has to be worked on heavily with any AOSP ROM due to having to find ways to get around the closed source drivers.
Customizable? I'll have to give an edge to some of the AOSP based Roms. Some such as Paranoid Android having a seemingly endless amount of options. Even GPE with gravitybox can't compare.
All in all its going to boil down to what you prefer.
AT&T HTC One M8
Google LG Nexus 5
AT&T HTC One M7
AT&T Moto X
AT&T Samsung Galaxy S II
Google Asus Nexus 7
AT&T HTC One X
Apple iPhone 1st gen
Apple iPhone 3G
Apple iPhone 4