Well, buy them both is what I did
Well, the Fenix P3D Rebel one and the OLight T20 Cree Q5 anyway.
The OLight has a sharper defined inner circle, the fenix is slightly fuzzier.
The OLight
seems slightly brighter but that might just be the fuzzy versus sharp. The Q5 Fenix might make that a non difference or bump the Fenix slightly higher.
I, personally, like the way the OLight shifts brightness levels better, the head is always screwed on tight. The Fenix is loosened to get some of the levels.
The fenix is the same diameter from the tailcap to the end of the head. The OLight has a larger head on it.
The OLight is ever so slightly longer, but not by much. Not enough for me to care in any case.