Jim,
My comments on your options...worth what you paid for it. ; )
1) Glareshield Mount - this is a great place from a reception/ease of wiring standpoint, but I would not recommend it. Installations I have seen with this mounting have a large glare that reflects off the canopy from the shiny antenna.
2) Under Cowl - I think this depends...are you planning to use a WAAS receiver (430W, etc.)? If so, I probably would NOT mount them there. There is significant RF shielding to the back of the antenna (big metal fuselage back there), and the WAAS reception is very sensitive to satellite mask angle. I know many people will post positive experience with their old NON-WAAS installations in that location (maybe it's OK), but I think people will have a higher likelihood of problems with the WAAS reception.
In the end, mine will likely go on top of the fuselage and I will pay the 0.15 knot penalty. ; )
Scott
7A Finishing