I can only speak from experience which is limited to the G0513X and X2. I have the "X" which was a 'tweener' model before public opinion drove Grizzly to cast iron trunnions. Seems this was the only thig anyone could find to criticize the saw about so they simply removed the possibility; good thinking. Both the steel and cast iron trunnions use the same hardened teeth for the table tilt and all other specs are identical or were last time I looked. Dad has the "X2" and both of us are very happy.
If I wanted something more, I would go for more horsepower before a brake or larger throat but, in the current offering you have to go to a 19" to get more HP unless you are willing to make the jump to the 636 series of 5HP 17" saws. I have only slowed my saw down once cutting hard maple at just shy of 12" (and being impatient). Essentially I heard the motor pitch change and backed off just a smidge and went ahead and finished my cut. The importance of more power, a foot brake or an electronic brake will depend on what the tool will be used for.