Tranny Cooler Help

I need a trans cooler since my new radiator doesn't have one built into it.

I am looking at the Derale 13740.
http://www.summitracing.com/parts/der-13740/overview/

This one comes with -6AN fittings.


However, they make the same cooler with -8 or -10 fittings.


Which size should I get?