I'm probably gonna buy a handheld. From my experience, they are very accurate. We used them to check people randomly on the field as their tanks heated up during the day. If they were hot, they got sent to the chrono station to adjust (with one of the big expensive red chronos). I never sent anyone to the station that wasnt actually shooting hot even when the handheld was only off by a few fps.
edit: I don't really know how well it will work with airsoft- smaller rounds to lock on and higher velocities...