what to get

Should i stick with 1 600w in my flower tent which is 4x4x6.5 or should i go with 2 400w ? not sure on which to get,

Generic question gets generic answer:

You have the potential to grow more grams with the 2X400 = 800 Watts

Kind of a no-brainer, spread out the light footprint and spread out the plants to occupy all the footprint and you will max your light potential.