On any exercise, try the Smith version and decide which works the muscles better, which is the bottom line. The pump & muscle stimulation are actually better in many cases. Also try 1/2 and 2/3 ROMs on the Smith, either for an entire set or as partials after reaching failure. The burn is intense, and IMO it's easier to progress knowing you can go to failure or beyond without risk, even less risk than free weights with a spotter.
In cases like close-grips and 1/2 motion behind the neck military presses I find the Smith and some machines work the muscles better than free weights. Try any exercise and decide-Yates found Smith squats much more effective than free weight squats. Free weights are widely over-hyped, and are not always as good. Another good one is seated behind the head triceps extensions on the Smith.