6-1/2" over 7-1/4" circular saw
The 6-1/2" circular saw is great for working due to its smaller size and weight over the larger saw. But you are limited to depth of cut that can be achieved. For example some saws this size will not cut all the way through a 1.5" thick lumber with the blade angled at  45 degrees. The lighter saw can be a real benefit for jobs such as cutting sheet goods on a roof job.   

The 7-1/4" saw will support a deeper cut which is particularly useful when making certain types of cuts. For example the Dewalt DW364 saw is capable of a cut depth of 2-7/16" when set with the blade in the normal 90 degree position. At a 45 degree angle this saw can cut 1-7/8" deep which is needed when cutting things like rafters that require cutting though conventional 1-1/2" thick framing lumber.