I like the phone not being in a case when I'm using it, so pouches and holsters work best for me.
I have a Shellster and find it ok. It's not the idea holster for me because the phone is in a shell when out of the holster, but I do like that it prevents the back of the phone from getting scratched up from the phone being placed on various surfaces. It's great when holding the phone vertically, but if I adjust the clip so that the phone is held horizontally then I worry about the phone sliding out. I found that at varying lean angles below horizontal the phone can slide forward in the holster and is at risk of falling out completely. It has actually managed to slide forward a couple of times, and actually slid out once; grant it, I was shoveling snow and taking garbage to the curb (i.e. involved in more movement than typical) that time. Now if I wear it horizontally I tip the holster so that the front end of the phone is angled up slightly.
Like you, I also have a leather horizontal holster. My third is a sleeve case, which is actually my favourite of the three, but I only use it if I'm going to have a backback or something with me (too bulky for a pant pocket) or don't have something in my jacket pocket. otherwise, the holsters are more convenient.