If you do, one thing I dislike about Siemens' implementation with the S7-1200 is that none of the onboard I/O is safety. So if you have a small application that would be a good fit for the 1200 and most or all the I/O is safety, you have to either abandon the built-in I/O, or go for the significantly more expensive 1500, in either the normal or ET200-SP (I/O terminal) form factor.
I guess that would probably not be a problem for you, since you already have low- and no-I/O CPUs.