HID's are always the best way to go. But the heat they put off is no joke. Make no mistake, CFL's make a lot of heat as well, but you can get them a lot closer to the plants... and in a situation where space is a serious issue, I would go with CFL's. You can virtually fill the tent with them and as long as you have some powerful ventilation, you'll be able to handle temps.
If you're lucky, you live in a cold climate that will easily help you combat temps. I personally live in a super arid place. Where it gets 110-115F in the summer. Battling temps is always my primary concern.
Go with a few high powered CFL's, and like 10 or so 23-26 watters to help you provide supplemental light on the sides and under the canopy. You won't be disappointed. I was growing 5 plants and I used about 17, 26W CFL's, and 1 55W CFL. So all in all, I was running about 500W of CFL's, and primarily due to the under-lighting I used, I had gorgeous results.
Here's some pics.
It was stupid complicated, but only because I was on a budget and couldn't afford all the things I needed to make it work right.
If you're considering getting a 400W HID, you can afford to do this, and do it right.
After vegging, I switched to the simplicity of a 400W HPS for flower, with a few supplemental 2300K CFL's for good measure. They were beautiful.
Sadly... this grow was busted. :\ Nothing to do with lighting obviously. But, things happen sometimes... Anyway, I'm not sure you'll have enough room for an HID in there, unless you do consummate LSTing and pretty much have them go 12/12 from seed.
Anyway, good luck with the grow! Be sure to shoot us a link to it so I can sub!