The Diablo is a pretty terrible choice- all it is, is a very expensive P7 torch with a nice mounting. Except that as the battery can't be removed, it's a very expensive torch with less functionality than a very cheap torch. Oh, and it's phsically bulkier, and only fractionally lighter than my P7 torches. The only significant plus it has is that the 3hr burn setting is better than the medium setting on my P7s. But then it needs to be as the full power is uselessly short for any decent ride.
No opinion here on the Nukeproof though, I'd like to see one in action. Also I'd maybe add Luu to your list. And whatever Troutlight is in budget.
I've used 2 DX P7 torches for about a year now, one with an external battery pack. Mates have 3 Bastids between them. I've had a very minor issue with one torch where the switch vibrated loose, just needed to be tightened back up, other than that flawless- and extremely durable. So I'd recommend that tbh. None of the Bastids have had any issues but I think I basically trust them less and I don't like the chargers much (my P7 chargers can run off an old Nokia wall wart). Fitting is easy, performance is great.