the 2 devhooks are
DevHook (Regular) - Emulate firmware 2.XX (soon possibly 3.XX as well) on a psp with firmware 1.5
Reverse DevHook (with HenC or HenD) - Emulate firmware 1.50 (homebrew) or 2.00-2.60 (GTA:LCS save data cheat device) on a psp with firmware 2.71
However; if you use lots of UMDs and homebrew, constantly having to use devhook can be a pain, so if you dont have a TA-082 motherboard you can use 2.71 SE-C with combines 1.50 and 2.71 into a hybrid firmware so you dont need devhook to switch between UMDs and homebrew