I understand your problem, having gone through a selection process for a dual-head SPECT system that could be adapted to do coincidence imaging of PET isotopes about 14 years ago. At that time there were about 7 to 8 vendors, so chosing between three is better, but still difficult. Have you gone on the site visits? If not, then put down everything you think you want in a machine on a check list and use it during the visits, that helps in remembering which system could do what. Also you might want to check out the chart smart for SPECT CT in our siste magazine Enterprise Imaging (formerly Advance for Imaging and Oncology Administrators). Here is the link. It was done in May of 2006 so some of the information may need updating, particularly the slices of the CT system.
http://www.advanceforioa.com/sharedresources/advanceforioa/resources/DownloadableResources/AR50106_p58ChartSmart.pdf
If you have trouble with the link, go to Enterprise Imaging and Therapeutic Radiology Management, click on the "Magazine" tab at the top left corner of the page, and scroll down to "Chart Smart" and click on the archived chart for May 2006.
Hope this helps and that others who are using the equipment will also reply to this post and let you know their opinions.
Joyce