For me (I am 6' 3" and 200 pounds), and it is a personal decision, I most often carry a S&W M&P Shield in one of these: G-Code Eclipse, Blade-Tech Revolution or Cook's adjustable clip kydex. When I carry my LCP AIWB, I have been using a Kusiak straight drop with mag pouch. I think this is the best combo for the LCP when carrying AIWB or Cross Draw. I suspect it might also be a great AIWB holster, with the mag pouch attached, for the Shield, but I haven't gotten one to review.
Weather and circumstance permitting, I may also carry my Ruger SR9c with the 17 round magazine, but it's not as easy to conceal, hence the need for somewhat heavier clothing or perhaps a jacket. It is of course heavier, but that's not always bad.
Generally, I think the best self defense carry pistol is one that has the basic form factor, especially width, and weight of the Shield. I like 9 mm since the ammo is not exotic and usually available, it is a very effective SD round with modern, factory hollow point loads, and the recoil is manageable for accurate follow up shots if necessary.
You might be interested in more on this on my self defense carry blog: http://sdcarry.blogspot.com
" onclick="window.open(this.href);return false; I will be publishing a review article on various "synthetic" (non-leather) AIWB holsters for the Shield soon. I've been using about five different models since last year and am about ready to write up my findings and preferences.
I would note that there is no "perfect" holster. Depends on the individual's preferences and circumstances. My ideal holster is one that carries the weapon safely and securely in a place on the body that is readily accessible in most circumstances and conceals reasonably. For me, I have migrated to synthetic holsters because I think they best represent those factors, and I like the option to re-holster with one hand if the other isn't available.
Hope that helps, but remember, it is only my opinion. You can read all the blogs and reviews out there, but you will have to get a few holsters and try them out for a while. I'd recommend starting with used holsters, if they are in good shape, since you will probably go through a number of them before you end up keeping two or three as your favorites.