I started my flight experience using 2 motorcycle batteries from aircraft spruce that would both just fit inside the RV-6 battery box. I could start the engine on either battery alone, but used both for starting. I was disappointed in the lifespan of those batteries and the need to vent the batteries.
I switched to the Odyssey PC680. Unfortunately, 2 would not fit in the battery box, and the PC-680 was also the biggest one that would fit the stock box. so I flew around with just one, and it worked great. Only problem was a lack of reserve energy. If I had not flown in a long time or had been running any lights or equipment, the battery would drain in short order and need to be recharged before it would restart the plane. I also became concerned about the total amount of energy available for alternator out scenarios.
I have currently settled on a sealed lead acid from Panasonic. It has 28 amp hours and weighs 24 lbs. and just fits in the RV-6 stock battery box.
It has qualities similar the the Odyssey Batteries (sealed, mount any position, deep cycle) except for the high cranking ability of the Odyssey's. Sam Buchanan had good results from a similar but half sized Panasonic battery in his RV-6 (a single one costing only $25). I have no flight experience with it yet. It was a little expensive at $120.