Depends on your situation. Are there any height requirements? If you're unable to hang it 12" above or more you'll need to know that. The thing to look at is the par charts on each... choose from those numbers. Look at the light spread. Remember that par numbers at 6" will be a 12" radius, those numbers are 6" from center.
I'm likely getting 40 degree so I can hang it higher. That way the light will come more from slightly more above than at an angle towards the walls of my tank, but I have no height restrictions.