Yes, it depends on which kind of birds we're talking about- if you're talking about the standard broilers, (Cornish X's,) then definitely go with a tractor. If you put even a few of them in a coop, it will literally be caked with poo and awful to manage, almost certainly guarenteeing that you won't do more than one batch. With a tractor, you can easily manage 25-50 in a small area, provided that you move it as much as possible (at least once per day.)
You can get breeds that are OK for meat and eggs, but you'll probably be disappointed with the amount of meat you get from them- for a decent meat yield, you need hybrids, like the Cornish X, or Colored Range Broilers. The only problem is that these aren't self sustaining- you can't breed them and get the same birds. If you could, you'd be a millionaire in about 5 minutes, as all the hatcheries get their broilers from a few places where the actual hybridization method is a closely guarded secret. Also, broilers usually die, either by the knife or naturally, long before the reach egg laying age.
If I were you, I'd continue building your 12x12 coop- and still divide it in half. Put your layers on one side (heck, even experiment with breeding a couple of meaties if you want,) then keep all of your equipment on the other side. Then, for about $150, build a tractor for your meaties. A great one, that I keep 50-75 in at a time, can be found at
www.pvcplans.com. It's light, easy to build, and very easy to move. My first tractor I built so heavy that it was a chore to move, which was a mistake.
Good luck!