If I had to choose one to carry with me, it would be a Pistol-grip with a long hose, because there are a lot of little fittings that you can't get to with a rigid tube and where you need one hand to hold the connector in place.
I keep Lever and Pistol styles around, and use the right one for the job at hand!
If it's a well-maintained machine that just needs a couple shots of grease a day, or a fitting in a hard to reach spot, the Pistol is the way to go.
If the machine hasn't been greased in a while, sometimes you need the extra pressure of the Lever to get it moving. If it is going to take a lot of grease because it's dry, or I am flushing the old grease out, the lever is nice. Prevents hand-crampitous...
I usually replace the grease fitting connector with an adjustable "heavy duty" 4-jaw one so I have less trouble with it popping off or leaking. Be warned though that if you put it on a recessed fitting that has a lot of back pressure, you may have a very hard time getting it to come off!
I like a long hose on the pistol gun so I can rest it on my leg or a tire or something while holding the fitting on. You can use your arm muscles instead of just your grip muscles to pump then.
The Lever gun has a rigid tube so I can lean into the high pressure locations.
My top pick would be an air-powered gun running off a drum with a long flex-hose, but those are hard to throw in your tool box! I someday might get an 18v gun, but it hasn't reached the top of the list yet...
My Tip O' The Day:
Leave the hose/tube screwed in hand-tight.
When changing grease tubes, after installing the new tube, remove the delivery hose and pump until grease from the new tube comes out (you will probably get some grease, then the air bubble will come through, then the fresh grease). Then screw the hose back in. The hose is so tiny, and the valve at the end of the hose so tight that pumping the air out can be difficult. Removing the hose gives it a straight shot. Leaving it hand tight allows you to re-bleed it when you hit that mid-tube air bubble!
Ben~