Last summer I did a 16' x 24' trex deck. Bought a 6.5 Fuel circular saw after having my 18v 6.5 Makita dog out doing the demo. Full disclosure I was already bought into the Milwaukee system the previous year and the makita was going on 6 years old.
I have 5 4.0 battery's running my drill and impact driver and the circular saw. The saw worked like a dog - rips, cross cuts, miters it chewed out the work. More than the benefits of no cord was that it was lightweight, fast, and cut lumber off the pile with a speed square rather than working off a bench.
I had a table saw, miter saw, and corded circular saw handy but that 6.5 fuel did everything on the job with power to spare. The only recommendation I have beyond buy what you like and only go 7.25" if you need the depth of cut for 4x4's would be to buy decent blades with the appropriate teeth style and keep fresh ones on hand. Sharp blades equals easier cuts and longer run times.