Fair warning; you will get all of our opinions based on what we are using. And most of us don't have much more than a sample size of 1 or 2 so figuring out what is the best one is going to be difficult.
I feel that the best IWB concealed carry holster is the Milt Sparks VersaMax2...if you don't mind spending some money and waiting a while. But nothing beats a really well made leather holster!
45 is only one different in size. If you get or have a 45 try it in your 9/40 holster before buying another holster. With a little retention adjustment my 45 fits fine in a couple of my holsters bought for my 9.I'm waiting on if its a Shield 45 or 40 or 9mm. I believe each one has a slightly different size than the other. I know for fact my 45 doesn't fit 40 or 9mm Shield holsters.
Thanks for the tip. Its hard to find a good 45 shield holster. All I see is 9 and 40. Maybe by the 40 and see how it fits my 45 shield.45 is only one different in size. If you get or have a 45 try it in your 9/40 holster before buying another holster. With a little retention adjustment my 45 fits fine in a couple of my holsters bought for my 9.