What are you going to do to keep the temps in check?
I am thinking the 250 will be ok. Less trouble making things more fun.
Is this your first grow?
I know at first I recomended the 400 but I am kinda second guessing myself now.
Mabey knowing a bit more about your plan would help us decide.

edit: I also did my math wrong. lets do this again:
100cm=3.2ft
50cm= 1.6ft

3.2ft x 1.6ft = 5.12 ft^2

250watt light ----------> 250watt/5.12 ft^2 = 48.8 watt/ft
400watt light ----------> 400watt/5.12 ft^2 = 78.1 watt/ft

ok cool, thats better I hope...
I guess doing it in my head I was kinda close.

I think I am still kinda wondering what your going to be doing for cooling?