For anyone who reads this thread, I ended up getting a 5D4. No regrets. It's not exciting, but it performs. The AF is impressive — I was able to reliably track people walking with an 85 1.2 II at f/2.5. High ISO noise performance seems to be about 2/3 stops better pixel-for-pixel. I find that 30mp isn't a huge increase but at least it's an increase. I guess my favorite thing is being able to lift shadows ~2 stops at base ISO with no real IQ penalty. Didn't think I'd care about dual card slots, but not having to hassle with backups in the field is actually great. Little upgrades like GPS and more AEB options are nice. Durability and weather sealing instill nothing but confidence, as I carried the 5D4+35L II around in moderate rain all day with no issues. Battery life is worse than the 5D2, but a full day's use is still possible on one battery. Canon needs to do better sensor-wise, but I wouldn't trade performance, reliability, QC, and CS for megapixels and DR.
Keep 5D2



