The short answer
Pick LaserGRBL if you have a diode laser with a GRBL controller and a Windows PC: it is quick to learn and does the job. Pick MeerK40t if you have a stock K40 with an M2 Nano board, a Mac or Linux computer, or several controller types in one shop. MeerK40t can do more, but it asks more of you, and its maintainers say the project is in maintenance mode, so fixes arrive slowly.
- Diode + Windows → LaserGRBL
- Stock K40, Mac or Linux → MeerK40t
- MeerK40t: more capable, more technical