I really like my 2 x 250 [2x96 vho act] on my 36 x 18 ... if I were to go to 36" front-back I'd probably suppliment with some T-5 in front/back, keep a pair of Halides [like on LA reflectors, as above ... perhaps the minis].
With 2 MH, everything between them is colored everywhere ... IMO you'll likely want to use something to keep the front/back from being much dimmer - given the success I've seen with T-5's - I'd argue for mixing a couple of those in, keep the power bill down.
When I dream of my 36 x 24/30 [vs the 48 x 24] ... I keep my 2 250's, keep the VHO actinic, and add 2 or 4 T-5's. Keep the heat/Halide running time down, just about perfect coverage.