Dear Mark,
Do you need to choose between the two? CimStation and Virtual-DMIS both use DMIS as their programming language, and any differences should be minor.
You could use the two to compliment each other. I have not used Virtual-DMIS, so I'm not able to give specific advantages/disadvantages, but Virtual-DMIS is intended for both on-line & off-line programming, whereas CimStation is specifically an off-line programming package.
I would advise that you investigate the possibility of running both softwares together for a time. This will be the best exercise you could undertake to assist your decision.
If you'd like to chat further, then drop me an e-mail at andrew@acel.co.uk.
Best regards,
Andrew.