There is a Key difference between the two. If you get XM you will not get all the sirius channels. They make you pay for a "best of sirius" package. The same for sirius, you need to pay for a best of XM package. Look over the channel line up and see which best fits you. I haven't gotten a unit for years and when I did get one I wasn't impressed. XM had the best tech back in the day I'm not sure if that is true anymore. If you are just going to use it in the car it really doesn't matter. Getting XM for my car was great when I drove for 3.5 hours a day.
Another way to go is see if you can get the online package and use your smart phone to play the app while your in the car. Haven't used the app yet but that might work and save you money.