Hi,
Personally I would go for the bigger one so much better for room. The price is not much different than the other go for a tall one as well if you need the extra room for the width and the height. The room is there. Everything you need in the room has a nice bit of space and you never can have too much space. Imo that is peace
I use 480wts in my 4x4 and I have there dimmer set at 50%. Through the veg stage. It's slowly increased to the 75%. As I approached the flower stage where it will never go above the 80% level last time I increased the level to the full extent of the weeks 4 to 8. And I have been getting the same results from the lightning being at the Watts I have been using it works out about 120wts less than the 480w and that's 360wts I think the most likely advice is 30wts per square foot and that's enough for flower less for the veg period. That's 480wts for the 4x4. People do it with less than this. But I wouldn't want anything less than 360wts /400wts. But I have no idea what is happening with the newer lighting on the market now and the improvement in their efficiency is likely to be better. The only thing I would say is that the price for not having a proper amount of light is a bit of a disappointment for the effort and the time you've spent.. So I would be making sure that the plants are doing what they should be doing. That's my penny's worth anyway. Good luck 
