I have a Speed-Six in 9mm, a LCR in 9mm, SP-101 in .357 and enjoy them all.
Yes,, .9mm ammo is readily available, and often cheaper to buy just for the primary purpose of range work. And yes,, they make several good choices of self defense ammo for the .9mm as well.
And yes,, they use moon clips.
The easiest way to load ammo into a clip, and unload the brass from a clip is to have the tools. A mooning tool is usually purchased,, and a de-mooner can be bought or home made.
I own a total of 5 different revolvers that use moon clips,, and have never had any second thoughts or worries about using them. But many feel they are just another thing that can go wrong, get bent, or otherwise they just don't like them.
A .357 doesn't enjoy the ammo availability as much as the .9mm currently does,, but it's not too far behind. Yes,, ammo for it can cost more. But the .357 can use .38 shorts, .38 spl, and .357 Mag. Very versatile. And plenty of self defense ammo out there too. No moon clips are necessary,, and if a speed type of reloading is desired,, there are speed loaders available. BUT,, they add a lot more bulk when trying to carry them. No tools necessary to load or unload the speed loaders.
I think the .357 is a better all around choice especially if you are a handloader. The .9mm has more limitations than the .357 does in that respect. Lots of bullet choices, up or down, heavy or light, plinking or even hunting, (even though you don't plan on that.) You can shoot very light .38 spl for a lot of fun,, or if you want a bit more of a fire-breather a heavy .357 max load can give more than many need. The .9mm bullet designs are fewer due to the necessity of them needing to be fed into a semi-auto for most people. The bullet designs for .357 are vastly different.
But since you've mentioned that the primary use will be just plain range work,, again, I'd still select the .357. Due simply to the availability of very different ammo choices, and not needing any special tools for the moon clips.