DOS.
Both will run at the same quality and resolution. The problem is emulation- there aren't many emulators out there capable of running Mac OS 7 (there's only really one- Basilisk II, and I don't think it is actively maintained anymore).
DOS, on the other hand, has a very healthy community backing it these days in the form of DOSBox and Boxer.app (which you can get at
http://www.boxerapp.com, Boxer is based off DOSBox with a nice native Mac GUI bolted on top). Both are actively maintained and performance is stellar (faster then the original system it emulates).
You can run the DOS version of SC2000 under DOSBox no problem. Heck, you can even track down a copy of Windows 3.1 and run that under DOSBox, and run the "Windows" releases of SC2000 (plus the Urban Renewal Kit editors) under that. It all works just fine.
-SC