My tank is 96"x36"x27" high. I have 4 250watt Halides, supplemented with T5 and LEDs.
If I were you, I'd go with 250watt as opposed to 400watt. My tank can grow and color up acros at every level of depth. I don't think you need 400watt for 30" depth, it's a lot of extra heat for I'm not sure much benefit. My ballasts/fixtures also support 400watt, so you could just get electronic ballasts with support for both 250/400, start with 250watt bulbs and then can always upgrade to 400watt if you need more power.
Then I would supplement with either T5s or Blue LEDs strips to your viewing tastes. Blue LEDs bring out colors in corals that nothing else will, but they also create a much more artificial look than blue T5s. I think Radium halides + Blue LEDs creates a beautiful yet electrical look to the corals, but everybody will have different taste.
Basically, halides aren't obsolete but they do have cons compared to LED. For smaller tanks I think LEDs clearly win, but for a tank your size the advantages of MH make them a real contender. At some point it will be personal preference. If you play it out over 5 years, the differences in cost won't be all that different so it really will come down to whether or not you prefer the look of LEDs vs the halides, or you have some other major consideration to worry about to tip the scales on way or the other (upfront cost, heat, etc).
Edit: I'll also add for my tank, the lights are on 12 hours but the halides are only on for 5. I think of the halides as the engine to grow the corals and not necessarily the light which is defining the look of the tank. With LEDs you have more ability to keep color consistent if you want to vary the intensity. With halides they are either on or off.
