OK I will get a little serious here. These guns are electronic devices that can, and do have problems. I know this since I have a Stalker and use it often and have seen variations due to set up problems, battery charging, improper angles, picking up bats, batted balls, reflections off of metal, even pointing it and pulling the trigger correctly. Scouts & pitching coaches who use these things every day have all this figured out and know how to use them properly. They also have their devices calibrated regularly. For everyone else, (myself included) they can be a useful tool, but can also be inaccurate and cause problems with pitchers trying to throw to the gun instead of pitching, and parents worrying about how hard (or not) little Johnny (or big) is throwing.
I use it as a way to measure the long-term development of my son’s velocity and more importantly the speed variations he has throwing his various pitches. I also use it to game chart pitchers for the HS coach so he can discuss with his kids how they are doing and developing and what they were throwing in a particular game. We also use it as a way to scout competitive teams and see what they are throwing at us.
We never give velocities to pitchers while they are in a game. Only the coach and his staff get the report. If someone asks me how hard so and so is throwing then I usually just give the working velocity of his FB. They are great tools but can also be a distraction.
As far as the differences between the two guns the Jugs is a fine piece of equipment, they generally (as BBMan pointed out not always) read 1-2 MPH slower than the Stalker and don’t have the range. There are lots of threads here with more details if you want to scan through the history.