If I were you, I'd try to split the closet up into two parts--one area that's 30" to 38" x 18" for flowering, one that's 22" to 30" x 18" for veg. Put up a wall...you can get a piece of sanded plywood at a lumber store, and have it cut to fit your area for pretty cheap. Line it (and the other walls) with mylar, or paint them flat white. Install it as a divider (there are lots of ways that will work well enough). You can get 2 or 3 plants in the flower area, and theres plenty of room for vegging 6 to 8 in the veg area.

At any rate, here's the bad news...I'd go with a 250w pretty much no matter what. Even if you use the 400w, the dimensions of the room will keep it from being as effective as it should be. Lights cast their lumens in pretty much a squarish/roundish pattern. The farther you are from the lights, the less lumens/light energy you get. Because your room is so much wider than it is deep, the plants on the far edges will be much farther away and won't get adequate light. IMO, a 400w is great for up to about a 3.5' x 3.5' area. a 250w, on the other hand, is good up to about 2'6" x 2'6". With mylar or flat white paint, you can probably get enough reflection to cover an area that's 3' (or even a bit over) wide.

I'm a light guy, and my basic rule is anything over 4,000 lumens per sq. ft. is good; over 5,000 is really good. There are all sorts of theories about PAR lights and other stuff, but I haven't seen much to suggest that estimating using lumens isn't damn accurate. A 39" x 18" grow area is 4.875 sq. ft. A 250w HPS puts out around 25,000 lumens. That's 5,128 lumens per sq. ft. You'll also have fewer ventilation and colling issues with a 250w as opposed to a 400w.

The 150w? It'll be okay if you supplement it with CFLs. Set up a 30" x 18" flower area, and get a couple of hanging/clamp lights with Y-adaptors and 4 26 watt CFLs. That's enough space and light for two plants. 150w = 15,000 lumens; 4 26w CFL = about 6700 lumens. 30" x 18" = 3.75 sq. ft. That's 5786 lumens per sq. ft.

Side note...and please remember that all of this is just personal opinion and that other better, smarter growers might disagree. Pretty much every 150w HID I've seen has the ballast built into the bulb area. Ballasts generate significant heat (and weigh a lot for hanging, too). If you get a separate ballast for a 250w, you'll reduce the heat in your grow area...IMO a 250w bulb generates as much or less heat than a 150w bulb/ballast combo. That's a plus.

You can go to Discount-Hydro (my favorite light shop), and pick up a 250w Valuline ballast

Discount Hydroponics - ValuLine Lighting

Stuff on ebay isn't that much cheaper, and this is a switchable ballast that allows you to use either MH or HPS. It's around $180 delivered, which isn't that much more than the 150 you were going to spend for the 400w...and the 400w isn't ideal for your space. And you get a new, switchable ballast system.

If you want to stick with the 150w, make your grow area smaller and get the CFLs...it will probably be less than $40 total, added to the $70 for the 150w HPS. Like I said, an 18" x 30" area is plenty for two decently sized plants (3' to 3.5' or maybe even a big bigger). If you squeezed it, you could make an 18" 24" grow room and just use the 150w...it would probably be enough light to flower two slightly smaller plants. Good luck!