Everyone will have their favourites. The answer should be which has best support in your area.
PCS7 is not a PLC but a DCS, the hardware is a combination of Siemens Step 7 PLC's (S7-300 and/or S7-400) and PC's usually with a Server, Operator Station and Engineering Station as minimum. The graphics being WinCC.
Siemens have the S7-300 and S7-400 PLC's for medium and large applications and nowadays S7-1200 for smaller applications. The S7-1500 has just been announced as a replacement for the S7-300.
ControlLogix and CompactLogix are the Allen Bradley ranges for small to large applications.
Both are very good PLC's, the Allen Bradley range being the biggest in North America and not too small over the rest of the world but Siemens has the biggest world base.
I would start by finding your local reps and find out what support they have in your area and what they will offer with training etc.
Does the cement plant already have a preference.