150 would be better because of the heat the 400 would create. If you can set up some intake and exhaust fans, then go ahead and get the 400 watter.
If I were you, Id get a 250w instead - the heat wont be that different from the 150, and it provides better penetration. Anything under a 400w in that space though, and youll wanna line it with mylar so it reflects all the light. NO TIN FOIL!!!
Oh, and Ive mounted a 600w HPS on its reflector, and then hung the reflector by a couple of hooks to a wooden shelf before and its been fine. For better heat dissipation though, youll wanna get some chain from a DIY store, only costs about £1.50 a metre.