If I were you I'd start thinking ahead to your total setup. You can easily match the light output of a 250w MH with 4-6 t5's packed very dense. With this you're looking at a fairly even output with a width of around 2ft. 8 tubes can outperform a 250w MH, but aren't going to be able to match a 400w MH. Keep in mind that the ability to get t5's in longer tubes means an 8 bulb 48" fixture is going to be able to veg roughly 12-18 sq ft, which would take multiple HID's, and the t5 will produce less heat. However, part of this intensity comes from the fact that you can place your T5ho fixture very close to the plants (at least compared to HID). This means they are less than ideal for flowering (unless you surround your plants with them). IMO if you can afford to get a nice 6-8 bulb T5ho fixture AND an HID, and have the space to flower and veg separately, that is your best bet. If not, I'd go for an HID.
Something to keep in mind, don't skimp on the reflector/hood. Watt for watt a cheap ballast paired with an incredible reflector will provide more light to your plants than the most expensive HID in the world using a shitty (or worse, no) reflector. For t5ho's this means individual bulb reflectors. Most budget brands don't use them, and lose 40% of their output because of it.
Spectrum makes a big difference too, and it's much harder to get good veg spectrum out of hps, easy with MH/T5's, easiest to fine tune with t5's.