The main difference is the improved safety. The 2.5 requires very little movement to turn off the safety (more like pressing a trigger/button). The original S2 required more movement of your thumb to disengage the safety.
P.S. The more recent versions have a slightly larger rope hook too.
The newer 2.5 is easier to shoot. If you aren't careful you can cause the release to go off when you press the safety in with the 2. Since the motion is very small on the 2.5 this is less of a problem.
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