Free vs Commercial Prolog comparison

I am a student at London South Bank University and is doing a Bsc Project on
Prolog. Comparing Free and Commercially available Prolog's to see if the
cost of commercially available Prolog software can be justified for
commercial development projects: A case study to compare shareware and
freeware Prolog to that of commercially available products. I have been
collecting facts from different people and I thought this might be a good
place to find some more opinions to help me get to my conclusion. Looking at
things like the services provided, the speed it works at, intelectual
property issues, additional features, cost, support/help provided,
interfaces, ease of use, Reputation of the prolog, Does sizes of commercial
projects affect Prolog choices, etc. 
I also want users to please take some time out and answer a short multiple
choice questionnaire (8 questions) for me, about Prolog. I want to include
the opinions of the users to my conclusion. It will be much appreciated,
Thank you in regards. 
Follow this link for the Questionnaire: 
https://docs.google.com/spreadsheet/viewform?formkey=dHJVVkFyRk1qMFJsRTdJQTlBaWYzNEE6MQ

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