From my limited experiance with electricity, I have found around 75% is better becouse of all the power out's around here.
A 600w w/digi ballast starts up at 6.1 amps and has a running draw of 5.5
I would say not to use more then 16 amps on a 20, I agree you should run two circiut's in the room.
You will need more in that room by the time you are done.(dehumidifier, circulating fan, heat? and so on).
If you have a choice I would use a 600 just becouse of the more output for the minimal heat and weight increase. I dont know where you are but where I am at elec. was going to cost just a little more monthly for the 6's.(like 18$)
So that is the way I went.
On a side note..Do you like those axial fans? Have you used them before?
I got a couple cause they were cheap, could not move air worth a shit.But u are using a small closet, so, they may work.
My 2 Cents.
Good Luck and Good Grow
DMG