Huge variation of impedance calculators with same parameters?
I was somehow in the same situation recently but my configuration was a differential pair in a stripline mode. I tried different calculators and got different results.
After trying to understand how the computation was done by the calculators, I gave up.

I looked for the most recent equations about stripline impedance and made my own calculator in a spreadsheet. Maybe the result is not the most accurate or doesn't take into account some parameters, but as an engineer in a preliminary design, I had to understand the result to be able to see some trade-off. Reading some signal integrity books teach me that the accuracy of these equations (for stripline, but I think it's about the same for CPW), is 10 to 20%.

You can find some equations on CPW impedance here : http://qucs.sourceforge.net/tech/node86.html

To get a better value, a 2D solver is required (accuracy is under 5%). Look at Polar Instruments ( http://www.polarinstruments.com ), their 2D solver is pretty simple but quite accurate.
HFSS is a 3D solver, maybe too complex for a first guess of impedance.