Need to replace the batteries on our 23' Chap. No overnights, just runabout, skiing, day trip type activites. Need to choose between two starting batteries (which is what came with the boat-665mca) or two dual purpose (I think 640 mca)The starting batteries are $39.99 and the dual $49.99, for what it's worth. Any thoughts on which would be best? Or maybe one of each, and switch when anchored and listening to radio, etc? By the way, I lean toward the get the best deal and replace after 2-3 years, rather than pay a lot and hope for 4-5.