Michael,
   I've got 50 hours with the skywatch system.    During that time I did see one target disappear behind the tail for maybe 10 seconds and I did see the whole system go offline in southern california near a certain big air force base.  I suspect the latter was an interference issue that any unit would have.   I wasn't convinced from the 50 hours that the single antenna "problem" is a big issue in practice either.
   I do have the 9900bx (aka avidyne) in my IVP but not enough time on it to comment yet.  At the time I bought it the audio annunciation was superior to skywatch. (e.g. it says traffic + clock angle + high/low).  I'm not sure if skywatch has done the same by now or not.
   You also might want to check which company will support your installation in a IVP.   I know L3 is completely handsoff on stormscopes in a IVP airframe.

   Colyn