Regarding XM entertainment subscriptions, the first one is $12.95 a month and each additional receiver is $5.95 a month under their "family plan". (keep in mind that this is for the music part, weather is a completely separate subscription)
In the case of a portable XM radio, you can simply transfer the receiver from the car to the house, to the hangar etc, as long as each location has an XM antenna, so that saves the cost of an extra subscription. I do this, though it meant buying several receiver dock/antenna packages, no small cost, about $60 each, nearly as much as a receiver, but it avoids extra monthly subscriptions.
The 396 is different than a typical XM entertainment receiver, it uses a special dedicated antenna that includes the XM receiver in the antenna. (There are some home stereo receivers that can use XM with a similar receiver/antenna.)
Unfortunately this means it is more hassle to use the 396 as a portable unit, you would need to take the antenna pod with you, and assuming you might want to use the GPS function in a car, that would be two antenna pods.
It's my view that it makes more sense not to use the 396 as an entertainment receiver, for a several reasons. First, there's the portability factors I describe above. Second, I'm not sure I want to tote a $2,200 aviation GPS around with me in my car and back and forth to my house, just so I can listen to XM radio without paying an additional $6 a month. Third, it seems to me that using the entertainment menus on the 396 is a distraction, and inconvenient compared to using just a plain old vanilla XM receiver.
Why clutter up your navigation screen while stepping through a menu, just so you can change the music channel? It's way easier and more intuitive to use a dedicated XM entertainment receiver, mounted to the co-pilot yoke. Lastly, if the music is coming from the 396, there's no way my teenage music surfing daughter can work the XM radio. (I isolate her headphones from mine when she's surfing.) Since the music subscription cost is the same, why not use a dedicated XM music receiver? There are a bunch of very good, very small and cheap XM radios out there, here are two of my favorites:
http://www.walmart.com/catalog/product.do?product_id=4875743http://www.walmart.com/catalog/product.do?product_id=4227930