I think you'd do great w/ a 250 watt MH.

I was running 2 x 250s on my 120 cube, and honestly, in retrospect, I wish I had gone w/ either a 500 or 1000w in a lumenarc. in my 30 gallon cube, I have 1 PHX 20k on a 250, and I run 2 Actinic T-5s, and 2 Actinic plus (white Actinic) to balance it more towards a 14K color temp and everything seems to be doing well (outside the damage the HIATT did.)