The only advantage I know of shooting a SPL in a gun chambered for it is I have seen folks post chrono readings that show a SPL loses speed when shot from a MAG gun.

I think it all hinges on whether you reload. If you don't reload, get a 44 SPL. Factory 44 MAG loads are just ridiculous and not much fun to shoot unless you are a masochist. Precision One has a good price on 44 SPL practice ammo, and Underwood sells relatively inexpensive JHPs for it. There's no advantage to getting a magnum unless you hunt with it.

If you do reload, I think the one advantage to a magnum is you can reload light magnums that are still very comfortable to shoot in an N frame while being hotter than any recommended load for a 44 SPL. My current 44 MAG pet load is 9gr of Universal under a 180gr, which is perfectly manageable in an N frame, but much hotter than the 44 SPL which maxes out at 5.6gr on Universal. Now, you can work up an above max 44 SPL load, but many are not comfortable doing that.

That said, the carry ammo I use in my 3" M629 is Underwood 44 SPL, and I have to say I'd be fine with just a 44 SPL. In fact, the only reason I didn't buy one when I got my M629 is I wanted a 3" barrel and 3" M24/M624 have been getting spendy. I'll still pick one up if I find a SPL for the right price.