I like manual grease guns for occasional greasing. Power Greasers are great for situations where you have lots of grease fittings. If your greasing a backhoe, dozer, or heavy equipment every day a power greaser is the way to go. Otherwise it is a lot of money for a low use item. When I worked for a driller and go through 2 tubes of grease when I would grease the rig, a power greaser would have been nice. I hate locking grease couplers. On stuck fittings, they make a damn mess when you have to bleed pressure off to get them to release. A 4 jaw coupler will pop loose with excess pressure and you lose only a little bit of grease. Due to the odd places engineers like to put grease fittings I like a long hose on a grease gun. Locking couplers are a SOB with hard to reach fittings as they are hard to reach to disconnect. A hose and a 4 jaw coupler allows me to get into most odd spots easier and a little wiggle and tug and they come free. Never had an issue with the hose being too long, just too short.
My preference is for pump style manual grease guns. It allows for more pressure, and pistol grip style causes my hand to hurt.
Alemite 555E, Lincoln 1133 pistol style, 1142 or 1147 pump style are all excellent grease guns. I change the end out for the Lincoln G300 4 jaw coupler as it makes them grab the zerk better, the 3 jaw couplers that come with them ****.
Here are a few tricks I have learned about grease guns. To check the amount of grease left, just pull the T handle in the bottom out. If it only comes out a little then your almost out, if it comes out a lot, then your pretty full. When loading new tubes, just screw the head on two or three turns then use the T handle and rotate it until it engages the follower then tap it on something a few times, give the gun a few pumps to clear any air and then screw the head on tight. 95% of the time you will have no problem with air lock.
