I was debating the very same thing and some research swayed me towards the Turbo Twist. Lots of people have mentioned that the housing of the JBJ Submariner tends to be not so waterproof. Water and electronics don't mix so well and, as a result, many people have mentioned that the JBJ unit tends to shock people when they stick their hands in the tank during maintenance. One person said that they got shocked even with a grounding rod! Also, in defense of the JBJ unit, people have applied aquarium-safe silicone sealant around the power cord and that seems to have lessened the shock delivered by the Submariner somewhat but not completely.
The JBJ units are normally $75 (5w) up to $105 (13w). No idea which one your friend wants to sell you.
I personally bought a 9W Turbo Twist but it's still on it's way so I cannot say how good it is yet other than most of the reviews for it have been very good.