GPS for sure, given I lack depth perception (only see out of one eye at a time). Have two laser devices, but I have trouble keeping it still. Have watch and Neo Ghost.All I really use is front/middle/back measurements. Although I will from time to time take a mark for clearing a hazard, etc.
Initially used the phone appas but they can suck up the battery life.
I like the neo, easy to carry in pokey or have mounted an cart.