In arenas only Hand-mounted pyro rocket works, the nitro boots and the rest only work in Bg's..
Engineering ( at least in wotlk ) doest give any money at all..
Do it like me, take both, engineering is tons of fun, you get usefull tools ( mailbox, portable bank, teleporter, portable repairs), plus you get to blow up stuff if you are a goblin engineer and build trinkets/heads if you are a gnomish engineer.
And then there is Enchanting, where u can make loads of money from and get a pretty good boost stats wise.