I have 5 apparatus in my dept. equipped with pumps (topmount, sidemount, rearmount) and they all have their pros/cons.
Topmount - You get superior scene observation and out of harms way of traffic, but it is a tripping hazard getting down off of the pump(usually preconnects in the way.
Sidemount - One-sided scene observation, cluttered with preconnects. Ummm, no pros for this
Rearmount - 2 sided observation, out of the way of the preconnects, but I just dont like to stand in the rear of a big red truck with flashy lights on a highway when the motoring a$$heads are driving past not paying attention.
I have seen a great idea to it, the topsidemount pump... It is the best of both worlds and I have only seen 2 made. The one I recall is made by Sutphen.
Just my $.02