Halide bulbs in parallel or series?

[lastlight]

Things are really cramped above my tank at the moment and I'm thinking of rotating all of my halide reflectors so the socket assemblies run parallel to the length of the tank. I'd like to move my rear T5s inwards a little and can't currently.

Every store-bought fixture I've ever seen has the bulbs running parallel and people always seem to mount their retrofit reflectors perpendicular.

Is it the same thing? Reflectors are pretty much square so I'm thinking it will make no difference...