I believe you will have more control/ sweet spot using 2. This way you can cover more area with same/simuliar amount of light. Hope this helps. I am debating doing the same just w/400w and lil bigger space. Welcome and happy growing
depends on how many plants, whiole 2 will cover more area, as Give Me points out but if you are growing just one or two plants than a single bulb with higher lumens would be better having their "sweet spot" be more focal.