S7-200 = been discontinued
S7-300 = small to medium projects
S7-400 = medium to large projects
S7-1200 = small to medium-sized projects (successor to the S7-200)
S7-1500 = medium to large projects
This is a rough description of the individual systems, but the choice of system depends on the complexity of the control system, the size of "geographical" plant will be and the capabilities of the system.
So more information on the system, and we can give your a hint about what direction your choice should be.
Read more about the different Siemens systems here:
http://www.automation.siemens.com/s...n/brochure_simatic-controller_overview_en.pdf