If you're already invested in a particular battery system, I would just get one that uses batteries you already have. I've had an 18v Makita in my truck for over 12 years, just because that was the batteries I used on woodworking stuff. It's still in my truck and gets used when it's the best tool for the job. I've never needed a big one to run long hours. If I'm framing a building, I use corded ones.
edited to add for Jimmy that was posting at the same time I was: Hold the trigger on your saw that the brake is not working good on for 30 seconds and let it go. If that doesn't improve the brake, the brushes may need changing.