T5 fixtures are awesome for seedlings and clones. Especially starting seeds because you can put them within inches of the canopy for compact growth, without really worrying about heat. I'll use a 4 tube 2' T5 fixture for just about the first few weeks of growth. After that and for bloom it is HID\HPS all the way (as mentioned they produce more lumens\watt and enhanced spectrum). A 250 watt HPS puts out enough light to grow maximum couple (2) decent plants no larger than 1-2' tall, or a single bushier plant.
400 watts is great for several 2-2.5' plants for personal head stash. Also agree with Jogro that the dim feature is great for increasing output as the plants grow. A 400w system would definitely provide more penetrating and higher quality usable light for flowering plants. If you can dim the ballast and run it at 150 or 250 watts for starting seeds and early vegetative growth, this seems ideal. Especially if you only plan on running one cycle of plants at a time,etc.
One thing to consider, though, is that while you CAN dim a 400 watt bulb down to 250 or whatever watts, doing so is not ideal and will probably shorten the life of the bulb. You should consider buying a 150 or 250 watt bulb, or whichever you plan to use that best corresponds to the settings of the ballast. However, NEVER EVER run a lesser wattage bulb like a 150 watt on the 400 watt setting (doing so will surely blow the bulb and may harm the ballast).
Once you start running 250+ watts of HID light heat issues and ventilation will surely need consideration. Air-cooled reflectors are good to have with these lights, so you can just cool the reflector with a 6-inch inline duct fan if not something more powerful for the reflector and\or whole grow tent.
If you go for the 400w, you may still find a T5 fixture helpful in the future: say if you needed an additional light for starting more seeds while you have plants blooming under the 400w, taking clones, keeping a mother plant, etc.
