I've had my XM pioneer Inno (like the newer XM3pi) since they were the newest on the market. I love it but you have to know there are trade offs.
My Inno is the size of a small phone has an integrated antenna and tuner, it can record xm of the air and playback recorded XM and mp3s. You can listen to the same station the entire time you are driving no matter how many time zones you go through. You can pretty much choose your genre, I like 80s, BPM, electric area, classic rewind, Ozzy, book radio and XMNPR, sure beats the preacher radio , lame talk and conway twitty that is all that's available trucking up the eastern shore. BUT... here are the down sides
Even with the magnetic patch antenna it will lose signal in light coverage areas. Installed units usually have a more sensitive tuner.
Even running it off of the dock it has good but not great sound quality. Again the small size means less sophisticated amplifier tech.
I got my son a a portable unit one black friday for free after rebate, it had a monochrome screen and needed to be in a dock with this little tuner chip to get XM but you could record to it and play back that without being plugged in. He used it for a couple of years but we discovered a fairly unique jeep issue. Your XM dock will corrode if it gets rained on and covered with mud
Since he bought himself a sedan DD and I got back the Jeep I replaces his portable with an Onyx this year, the tuner is much more sensitive and the sound is much better.