I have both.
I have a 4" Model 610 No Dash. In 1992 I took the 610 by to see Ron Power at Power Custom. I had him do his Power Custom Combat work on the 610. I also had him shorten it off the back from 5" to 4". I had it in the shop again this Fall. He checked it over, and added an End Shake Bearing to snug it up a hair. Back to perfect again. End shake was not bad enough that I had even noticed it at all.
My 310 Night Guard is a great carry gun. I did change the grips on it from Goodyear to S&W checkered wood compacts. Mainly for the looks.
I have no problems carrying it all day in a Lobo Enhanced Pancake with optional Inner Hammer Shield. I carry at 4 O'Clock. I normally carry 1 spare reload infront of the holster in a California Competition 7-Shot 38/357 (Long Post) Moon Clip Holder. Some times two. I may go to a North Mountain single post that holds 2 stacked in the future.
I guess you know all the important things about a Night Guard?
The 310 NG is an airweight N Frame with 2-1/2" barrel.
The Front Sight is an XS 24/7 Tritium Night Sight.
The back sight is a Cylinder & Slide Fixed Extreme Duty with a round sight notch to match the round XS front sight.
It will shoot 40 Smith & Wesson as well as 10mm interchangeably.
Recoil is brisk with full house loads, but no worse to me than a K Frame shooting 357 Mag 158g loads.
Ease of carry. It must have to do with weight. I also have a 3" 625, and the 310 seems smaller when carrying it. This has to be a factor of the 310 being so much lighter. I have several concealed carry shirts. The 310 goes away under them, with no problem for year round use.
Ok now for the Bad Points: I guess I will need to keep working on that, my list is blank on bad points.
Smith & Wesson got the Night Guard Series right the first time. In my opinion they should have kept the 315 and 310 in production longer.
I normally carry 175 Silvertips or some 180 Cor Bon in both for town. Mostly shoot 40 S&W reloads for matches and plinking.
I conceal carry the 4" 610 some also in another Lobo Enhanced Pancake.
I have no idea why the 310 NG was not more popular than it was. It shooting 40 S&W should have been a great selling point if nothing else. While the 10mm is not a real popular round, they can be ordered or found on the shelf here and their. The 40 S&W is available everywhere in cheap practice, as well as carry ammo. I shoot mostly 40 S&W reloads through both of mine.
I really like the 610 and 310 NG. The next best thing to a 310 NG would be a 325 NG, followed by a 386 NG or 327 NG. I really like the Night Guard Series for a concealed carry revolver.