Hmm, I'm not sure what the question is here... do you have a really good deal on a 2x 400W halide set-up, and you're asking if you should buy it or not? If it's a steal and you have some disposable funds, buy it. You never know when you're going to find a good deal on a 36" deep tank, ya know?
On the other hand, I wouldn't consider anything over 250W for a tank 18" deep... halide has really good penetration at depth and you may find there's too much light for a mixed reef.
Do you have issues with your current T5 set-up over the 80 gallon? Not enough light, or... ? T5s may not penetrate as well, but if you're still aiming for a mixed reef in the 24" deep tank you may want to consider going that route, since chillers are expensive and add significantly to your energy bill... MH is great, but projects a lot of heat. You'll still be able to keep the high light stuff in the top 1/2 of the tank, and have lots of medium light places for things like zoas and palys (low light stuff like mushrooms can be tucked in under ledges and such).
If you want clams on the sandbed in the 24" deep tank, go for the halides. And get a chiller
