HI Dave,
Although not an owner yet, I hope I am not out of line here...
I have done a fair amount of research into what mount to get myself, which is the EQ6 with Synscan. A few things that pushed me towards this mount these are, the load capacity, although the figures state 40lb weight limits it has been proven to take a little more, in some cases up to 50lb. Although typically these values would only be good enough for visual use, and also the tube length has a great deal of say in it too.
Pointing accuracy is only as good as the polar alignment and the stars you use for aligning with, many opt for just a one star alignment close to the object you want to view and then goto it from there, the accuracy is said to be very good to excellent.
Tracking, well I havent heard many complaints about its ability to track, although that said typical periodic error I am lead to belive is around 11 to 18 arcseconds peak to peak. With some mounts better. Older mounts without the synscan controller do suffer in this field.
Depending on what you wish to use it for the eq6 will handle the Mak OTA no worries at all, as long as you have a suitable dovetail, I didnt like the look of the one that came with the EQ6 so I have organised to have one made up for me later in the year.
All the other mounts that you have mentioned do have their pro's and con's too. I steered away from the sphinx it just didnt look like it could handle the weights (over 30lb for me) The HEQ5 is a very capable mount with its load limits of around 25lb but also limited with weights. As for the others I am unsure, it was suggested to me to get a CG-5, but the limits also put me off.
At around $2300 for the EQ6 with Synscan and a 40+lb load limit you can go wrong. Check out the yahoo group on the eq6 too, loads of info there.
http://groups.yahoo.com/groups/eq6
good luck with your choice.