For practicing reaction time, I would recommend an actual practice tree over software or phone apps. Software and phone apps are not guaranteed to have 1ms accuracy due to operating system constraints. They are okay for getting in the ballpark though.
I really like to practice in my race car. I use the Portatree Eliminator Super 2000 (but the Pocket Pal also works) and a vehicle connection. My practice tree plugs right into the car, and I can sit in the car at home or at the track and practice between rounds
.