1/2, 7/16 and 3/8 in that order are most common, common users won't notice the difference in performance..
Most use compression fittings but its all up to your taste, how do you want it to look??
unless your running an extreme loop most pumps will do just fine .
EKWB makes crap and not just with the recent run of plating, I wouldn't recommend any of their products......
if your in the US take a look at
PCperformance,
FrozenCPU , Aquatuning those stores will have a good supply to mull over .