I think the XM antenna is bigger because XM has fewer satelites and they use repeaters. Sirius is all satelite and I don't think you can use them with XM.

You can use the antennas in the cabin. That's where they put my XM antenna. They go through fiberglass. Our local installer puts them inside all the time. You obviously can't put them under metal obstructions or more than just thin fiberglass and still get a sat signal. Having said that I would use the new 1" x 1" antenna from XM outside since they are now so small.

As far as Sirius having 3 satelites against XM's 2 sats. That's because Sirius uses different orbits and has to have 3. Only 2 are overhead at any one time. Xm's 2 sats are fixed. Their design doesn't require 3 satelites. The repeaters are used in downtown areas to insure you don't loose signal in a parking garage etc.

XM's current promotion for a RoadyII that comes with the very small antenna and installation kit is Buy 1 and get one free by mail in form. I got mine at Best Buy for $93

Notice in the link how small the antenna is.

http://www.xmradio.com/roady2/index.jsp

I have my Sirius antenna mounted directly under the deck at the nose of the boat. It works fine and I have full signal strength.

I got a little XM antenna to go with my alpine head unit and it is not much bigger than a quarter and it works great.

I ended up doing the deck mount. I am getting great reception. It goes with the GPS Speedo Ant. and the GPS Ant. Dave

I got mine behind the gauges under the fiberglass fairing. I get full signal no mater what. Sirius satlite