I've been through many of them.
Altamont's boot - hard square back that hurts under recoil, "ears" bite the web of your thumb, hard to grip with sweaty hands
Altamont's Altai - too large to conceal in pocket. A round to square conversion type. Great otherwise and an outstanding grip for a 3 or 5 inch model 60
Hogue's bantam (the factory grip if I am not mistaken) - doesn't work with speedloader well, rubber is stickier/softer than I like, and it is a bit too slim
Hogue's monogrip - just a bit too large to conceal. A round to square conversion type. Might be the best choice for a J-magnum snub like the 340.
Altamont's combat grip (the one that looks like a devil horn)- not sure what the thinking is here. No checkering, slips all over the place, too large to conceal in pocket, not as stable as a conversion type. It looks "cool" I guess.
Pachmayr's Gripper (not the pro gripper) - too large, fit is poor.
Hands down I prefer the Uncle Mike's Boot grip (no longer made) for my 442. They are small enough for pocket carry. It is made of a harder rubber and is checkered for extra grip. There are thumbrest cutouts on both sides and they allow for an actual grip with your thumb. It has an open backstrap that fits tight, round back, and rounded upper edges so under recoil the web of your thumb doesn't get "bitten." They also permit use of a speedloader (both HKS and Safariland work). The grip is roundish and slightly thicker than the Hogue bantam and the rubber is much less sticky. The rubber is just perfectly balanced between recoil absorption, able to provide grip with sweaty hands, providing enough grip, and not cling to everything. They may not be as pretty as wood. But functionally these are hands down the best. If one carries a J-frame 38 snub seriously, I think one should really consider buying one.
I found out some of the history (which may be Fuddlore, but it makes sense) that the Uncle Mike's Boot design came from Craig Spegel who still makes the grip in wood. Apparently he licensed the design for a term or had some sort of royalty agreement with Uncle Mike's and after a while the deal wasn't found acceptable to either him or Uncle Mike's so they ceased production. This is unfortunate as they are still making J-frames and not making these grips anymore. Also, to me the real Craig Spegel's don't cut it...being made of wood, which in my opinion is an inferior material for an everyday carry grip being subjected to sweat, being used by sweaty hands, etc. Wood looks nice. And it's fine for a occasional use firearm or one with a larger grip surface. But hard rubber is a superior material in terms of durability and utility on these little revolvers. It's also MUCH less expensive.