I generally use both a shoulder sling and a belt rig; I make my own holsters, and form them to the pistol they will be used with. I always carry my largest piece in the shoulder sling because, especially with mags or speed-loaders on the other side, they tend to distribute the weight of a big pistol mote evenly. A Ruger Super Red Hawk .44 Mag or a Hi Point .40 or .45 can really put some stress on your waist, affecting your gait and causing you to compensate for the extra weight, causing more back strain than I care for. Plus, as noted by others, you can draw from just about any position or circumstance with a shoulder sling, whereas a belt holster can get tied up pretty easily, is less comfortable (I hate having a big chunk of metal/plastic digging into my side when I sit a certain way). There are actually holsters on the market designed strictly for driving; I guess you change whenever you exit the vehicle.
Always anchor both sides of your shoulder sling to your belt; they tend to swing and want to go with the piece when you draw it, which can be, at the very least, embarrassing, even if it doesn't get you killed. Horizontal or vertical depends on the pistol; you probably wouldn't carry a 7" barrel horizontally, but an Officer's Model 1911 or even a Beretta M92 work nicely worn sideways instead of up and down. The higher you carry it, the better, as long as you can clear your armpit on the draw. I've also found a shoulder sling to be faster on the draw carried concealed; for me, belt holsters ride too high for a fast draw. I'm building an angled across-the-stomach holster right now; unless I miss my guess, this will be faster than anything. Think about Hickok's red sash. Not as concealable as some prefer, but definitely fast when built right.
Bottom line? It's a matter of personal choice, and i prefer a shoulder sling. Thanks. kbv