The 88860 has a slightly bigger head, and offers 20 more lumens (150 vs. 130).
IMO you won't notice the difference between 20 lumens in any direction. I'd stick with the 88850 because its (somewhat) smaller, (somewhat) lighter, cheaper, and it will still serve up plenty of light.
I would be too, it's very nice! That's what I am going for with my AR build. There aren't too many of us left that enjoy the 20" variety but man are they nice!