First, do you have any other modern cordless tools? Drills, impacts, reciprocating saws, grinders, lights, radios, etc.? If so, go with the same brand for battery commonality. If you already have the batteries and chargers you can save money by getting the bare tool. (This is excluding Craftsman. Don't buy new Craftsman power tools.)
If not and you want cordless, Milwaukee and DeWalt are the brands to go with. And between those it seems like Milwaukee has stayed a half step ahead of DeWalt for the last 10 years. But that may just be my perception.
If I could only have one circular saw, it'd be the Skil Mag-77 worm drive. It's corded and heavy, but nothing beats it. This one will last you your lifetime unlike the battery tools and won't pigeon hole you to a particular brand of batteries.