Laser or GPS? HELP!!!

I own a John Deere 120c excavator and do a lot of basements, Driveways, Shed pads, and Ditching... also a fair amount of culvert work. I am looking at finding a system to eliminate getting out and shooting grade with a rod. Something I can get into for a few grand and that's fairly simple to run. Not sure where to even start. Would like it to be versatile as then I could move it to my d5 dozer as well. any help is appreciated.. I feel lost with all the options.

Laser or GPS? HELP!!!