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The difference is just the material as you have stated David. They will all have a similar cap/spring setup to protect your firing pin. In my opinion they will all have equivalent service lives.

they both work the same in my experience the aluminum body snap caps last longer

If you are going to use them for clearing drills and dry fire, the aluminum caps will work best and last longest. The plastic body metal head ones get chewed up going through the magazine and action.