Small closet with no vents - get LED, however a 90w ufo will NEVER be equivalent to a 400w hps in terms of growing horsepower. Look to replace hid with at least 1/2 to 2/3 the wattage of a HID set up. To elaborate I used to use 2x 400w HPS plus CFL sides. I now use 600w of CREE 3w chipset LED output - no drop in yield and no heat issues. If you're looking for one light to replace a 400w hid I'd say take a look at the Pro-Grow 260 from Hydroponics Hut or a Spectra 290 from Grow Led Hydro.
Just do a lot of research before you buy and do yourself a favor and forget the 90w ufos, those are only good for side lighting in my experience. Get a 3w chipset panel and make sure you know the actual output wattage on the lamp you're considering as companies love to advertise the actual chipset capacity instead of the actual output of the lamp (I'm looking at you Gotham Hydro).