The following are the factors for considering a DCS:

1) Functionality - Seamless Integration between all control functions
2) Technology - Easy to use, simple to maintain, effective user interface
3) Interoperability - To other systems outside.
4) Implementation Process - Quick implementation Focus ( deciding factors here will be DCS vendors who have experience engineers,consultants,Project Management and a proven method of implementation)
5) Service & Support - Post Purchase Support (Who provides the best post purchase support , gives best documentation(Printed & Online), online group discussions, life cycle management without worries to name a few)
6) Training - Vendor training given to operators, Maintenance personell and so on.
7) Documentation - All standard and custom documentation of the project and interconnections
8) Viability - Strategy of the vendor, Marketing Management, Amount spent on R&D and so on
9) Vision - Future market focus, (to be truly a visionary,a vendor has to tie together all the characteristics of the industry needs)
10) Initial Cost - Cheaper/Costs of customization/Consulting etc
11) Ongoing Cost - Ongoing costs include custom enhancements,education & training of the end user.
12) Exit Cost - Cost landed at when the entire setup is complete.
13) User Experience - Excellent user references.