I've never used an electric gun,so can't give an opinion.Keep a Lincoln air gun on my truck,and have no complaints.Adjust trencher booms,excavator tracks,and it's never let me down.
The important thing is to have "A" grease gun.If you've never had to line bore a loader frame,or excavator stick,you have no idea just how important grease,of any brand,or grade,can be.A consistant grease schedule can make or break any machine.A serviceman who knows enough to clear debris from a zerk,before greasing,can be worth his weight in gold.
Overgreasing is better than the alternative,trust me.Blown grease seals only present a problem when grease isn't applied daily.Loader frames,excavator bucket linkages,and trencher pivot rings,need daily attention,and they get it,either from the lube man,or the mechanic,just depends on who the "BOSS" thinks is less expensive.