Go with a horizontal, especially if you're "big chested" (fat). MUCH easier to draw.
Given the choice between wide shoulder straps and narrow ones, go wide, the wider the better. This increases long-term wearability comfort.
Given the choice between slick finished leather and rough suede leather shoulder straps, go with the rough suede. It helps lock the holster in place and keeps it from sliping and sliding around the shoulders.
Side-straps and tie downs are a personal choice. I don't much care for them, but I do use them occasionally.
A lot of people say you'll never be comfortable with a shoulder holster.
I say BS to that.
Pick your shoulder holster carefully, and you'll have no problems.
I've carried a 4" Model 19 or a H&K P7M13 for upwards 14 hours in shoulder holsters without appreciable fatigue or discomfort.
About 20 minutes with an IWB holster, though, and I'm ready to scream bloody murder.