I have Sirius with a 1 year commitment it's also $10bucks just like XM. My wife and sister are added to my account for $6 bucks per month as well.
I went with Sirius because I prefer its Dance genre better. But your choice in service should be over format you like not what someone says is better. Both services are awesome IMO.

If I had a car with XM standard I would use it, and vice versa. My Accord didn't have either so I chose Sirius.

I do know for a fact some Sirius and XM users have swapped antennas and vice versa the antennas are near identical, but the tuners boxes are different. Rumor has it, Alpine is going to have a dual XM-Sirius tuner one day. Who knows?