1) Does the blade type antenna provide "better" coverage than the rod type? What about aerodynamics between the two?

I happen to have checked the pattern at 1M over the transponder band on a rami blade versus a simple 1/4 wave. They both seemed pretty close in gain and (non)directionality.



2) Potentially dumb question - Ground Planes -
Actually an really interesting question.

An outside ground plane should be ideal from an RF standpoint, but how would you do it from a low drag standpoint, and is it worth it?

I don't have CF there, but would still want to put the ground plane inside and hope it was good enough. Officially, it seems like it ought to be up to the av tech signing off on the install, but finding one that has a clear understanding of this seems unlikely. I guess you could try to measure the installed antenna pattern, but even if you did, what is good enough? Maybe a VSWR spec on the transponder, but CF seems likely to make loss which would make that ok. It comes down to antenna system loss. For the places you plan to fly, is it good enough so the transponder does it's job of keeping you safe. I would do what is easy, (inside ground plane) and then try it out and see if it works. Not sure if the FAA's ADSB report includes signal strength information. It ought to.