i think you should consider how much money you want to put in this project. tank size is a big factor.
ok im not sure bout how much gallons does an elongatus need, but im pretty sure its about 70 gals.(correct me if im wrong). as for a rhom, an adult would need a strict minimum of 120 gals. if you plan to buy a tank that big, you'll buy your rhom already grown up (they take several years to grow up to adult size) a 15" + rhom would cost you hundreds of dollars.
instead, why not buying shoaling fish like caribes,(like 4-5) they will give the show you are looking for on feeding sessions and they look mean too. but if you want something less expensive, an elong with a smaller tank and a good powerhead will be what your lookin for. for a bad ass looking p, the big rhom will cost you a lot, but will look mean as hell.
up to you, but it doesn't only rely on agresseiveness, think about tank size and money...