I have both saws and by far the brushless 7-1/4" is the better one. It has more power and using it for framing, does a better job cross-cutting and ripping lumber and ripping plywood and osb. It also has an electric brake and is constructed of more metal parts than plastic. For me it is comparable to a corded saw, but have a couple of extra batteries handy; like all cordless saws it goes through a battery pretty fast with constant use. I can usually cut two 12' to 14' stair stringers on one fully charged battery.
I normally use the 6-1/2" saw as a trim saw because it's lighter and with the blade on the left side it's easier to see the cut line.
If money isn't the main issue for buying, I would go with the 7-1/4" brushless, it's a much better tool.