Go for the 10- it will give you the flexibility to both BIG 5 gallon batches and moderate sized 10 gallon batches. With a 5 gallon cooler, your max grain bill will be about 15# if you do a thick (1 qt/lb) mash. If you are concerned about dead space and temperature loss, a two-inch thick piece of foam insulation laid on top of the grain bed will give you rock-solid temperature stability, IME.
Something like this:
Last edited by flyangler18; 10-13-2008 at 10:34 PM.