The SV1 can only stay in the car, its an alright unit. The unit itself has been discontined. I would get the InV is the replacement for it. Works of an FM transmitter, like every other unit. Although if you want the best reception I would either use a FM direct adapter, Cassette player or a AUX hookup, to avoid interference and static, with that unit and the other series that came out around that time, the FM transmitters are not as good as they use to be. The FCC got involved with people being able to get "free satellite radio" from driving behind someone else.
If you go through sirius themselves they have deals you can get free radios when signing up for months at a time. I believe they have one going on right now for the Stratus. Not 100% sure though.
With signing up for the subscription the units themselves have the SID (serial number) printed on the back of it, as well as before activation the unit itself shows the SID number on it.
If your looking for both you and your girlfriend are you planning on two separate units or 1? If you are looking at different subscriptions first unit is 13 the 2 would be 7 bucks a month if you pay monthly.
Units like the sportster, starmate, stratus are units you can actually bring between multiple vehicles as well as the home. Online access is free with any unit subscription.
Differences between the units
Sportster: color / bigger screen, turn dial, pause play feature with 30 minute record time, and game/song/artist alerts. Comes with remote, and window mount. Option to bring in home.
Starmate: smaller school to sportster, button dial, pause play feature with 30 minute record time, and game/song/artist alerts. Comes with remote, and window mount. Option to bring in home.
Stratus: Dash Mount no pause play feature, remote, 2 line display, but smaller then other units. Option to bring in home.
InV prob the unit closest to the SV1, dash mount, stay in car only. Single line display, bit of a pain to change stations from.
SVI: designed to look like a radar detector, could be mounted on window as well as visor, single line display. Stay in car only unit.