I have the one shaped like an egg. BTW, it is an XM system. I was having a hard time coming up with a good place for it that wouldn't look tacked on. The guy doing the install suggested the dash because he had done the same with his Civic. Turns out he war right.
From what I understand, it is pushed as far forward in the dash as possible to attain the best signal it can. The plastic on the dash doesn't have an affect on it at all.