We use both.
S7-300 : Depending on model, can be much, much faster. Certainly has a lot of stability in its firmware and software (can use SIMATIC or TIA v11 SP2). Can use breakpoints in development. Expensive.
S7-1200: Dirt cheap for the features it has. Requires TIA for development. Less stable firmware (v1.0 had some horrible bugs in it, most are sorted now). Fairly scalable but not nearly to extent that 300 is. Much newer. Did I mention it was cheap?