For years I have used a down bag with no zipper. Now I am in the market for a new bag and zippers are now located on the left, right and center. I am a lefty, so do I want the zipper on the left, or is there more to it, as in the location of the tent door for an easier exit?
The "accepted wisdom" is that you want the zipper on the opposite side from your strong hand, so you'd want a right zip in theory.
OTOH, all bets are off if you actually try a couple of bags out and find out heaven forbid, you're different than what the "experts" think. All bets are also off if you have a zip-together hiking buddy and you want bags that zip together.
Find a store that'll let you try out a couple so you can determine your preference.
Loc: Marina del Rey,CA
It all comes down to personal preference. I am right-handed; I tend to sleep on my left side; therefore, I like a left side zipper that I can reach with my right hand. If I have my bag partly open, I have my dominant hand free to grab things or turn pages on a book if I am reading. My tent door is behind me (at my head), so that makes little difference to me. I usually am alone, so that is of no consequence either.
A center zip makes little sense to me, but I am sure there is some justification for it.
I suggest going to a store like REI, climbing into one of each and trying them out.
Don't get me started, you know how I get.
I have both, and don't find much difference. I'm left-handed, but I don't think it's really relevant. One consideration might be whether or not you ever intend to use the bag with a bivy sack...you might want the zippers on the same side, assuming the bivy had a zipper. Or maybe opposite sides...I don't know.
Having thus muddied the waters, let me add another variable. I have a mating pair of old Trailwise Slimline bags, and the draft flap on one is attached to the bottom side of the bag, and to the top on the other. That way the zipper on the bottom is covered when they're mated. Not all manufacturers do this, maybe not any these days, for all I know. It's something to check for, though. Having the draft flap attached to the top of the bag insures that it will hang down over the zipper, rather than possibly falling away from the zipper. A thick, well designed draft flap minimizes this possibility, though.
I made my first mummy down bag with a zipper in the middle, found it the best solution: easy to get to with both hands, no zipper in your way when you sleep on your side, easy to keep open or closed to regulate the temperature. The con is that the top part and puller, if not properly protected, hidden under some kind of flap, can get into chin, mouth or nose quite uncomfortably; needs a good anti-draft flap too. Some bags have zipper starting on the side and ending on the front (diagonal?), probably a good compromise.