I've been carrying a Shield 9mm for a couple years now. It's been a great little pistol in almost every way. Comfy, reliable, reasonably cheap to feed, eats whatever ya have on hand, accurate, etc.....
BUT
I spend a lot of time wandering around in the AZ deserts and wilderness areas alone on my mosickle for 7 months each year and I HATE rattlers. I load usually a couple shot shells on top of the mag for this purpose and will go through two boxes each winter. Ya don't HAVE to use shot shells of course, and the 9mm shells don't have many pellets in em. They do feed just fine in the Shield.
I have used .45 shot shells , in longer barreled pistols and they are WAY more useful.
This makes me sort of want to maybe get a .45 Shield.... just mostly for the much bigger wallop when using shot shells. I think for normal SD use the 9mm is just fine and has a slight capacity advantage as well as being cheaper to feed at the range. I'll have to ponder it a bit waiting for the inventories to increase and the prices to decrease maybe....
I do like the .45 round in general ... but I haven't been able to actually try out a .45 Shield yet. THAT will determine a purchase or not. Maybe I'll end up with one of each.. ya never know.