I'm all about plastic for the Glock, and all about leather for the 1911. why? i really don't know. either way, they are all OWB holsters except one, and I don't like that one very much.
for the Glock, my first holster was a Safariland (I have no idea what model) and it is a REALLY awesome thumb-release plastic retention holster. I lost it for a while and bought a Blackhawk SerpaII and it is great as well, and then I found the Safariland again so I have both. both come with a belt attachment but I MUCH rather the paddle on both of them. Blackhawk gets a +1 because of the option to change the angle, Safariland feels more solid but you can't change the angle.
as for leather, the only leather holsters I have are for the 1911. the IWB holster is crap simply because it's uncomfortable. don't like it much at all. feels like a brick inside my waistband. I have two leather OWB holsters for the 1911, one only covers the trigger mechanism and lets the front of the slide hang out, and the other covers the whole gun down to the muzzle but I don't like it much because it hangs WAY FAR off my body. it looks "classier" but it's much less practical. Both are Galco. (Galco is GREAT! please don't read me wrong. I just don't much care for these particular two holsters, although they are QUALITY!)
all in all, my experience with plastic holsters is FAR better than my experience with leather.
