I have the Old Faithful holster and bought my as a quick assembly kit. Very easy to put together and very adjustable. Fit is very good. Using it with a Sig P229r. I did check and they have a kit for the M&P 40 full size. Delivery did take about 2 - 3 weeks would be my only complaint.
I have been an advocate for Kholster brand holsters ever since I got mine. Usually you can get them for about $40. The leather on them makes sure that no part of the gun touches your skin (in case you choose not to wear an undershirt, or even if you do sweat won't get on the gun). The kydex is formed for your specific model. The clips are very sturdy and they allow you to fully tuck in your shirt as well. I have always been able to quickly draw from it and have never felt it loosen (I've had one for apx 2 years now). It also gives you the ability to adjust it to many positions for various cants. Here's a link: