The Sirius S50 all the way. I just purchased one myself, although I have not installed it on my bike yet. I've got it in my car for now though.
Features:
Removable unit for portability
Built in 4GB of memory
Can be used as a portable MP3 player
Pause, fastforward, rewind and record songs and shows off the radio
Large, easy-to-use knob, which is perfect when wearing motorcycle gloves
Easy to read screen
Voice cues when navigating channels or menus so that you don't have to take your eyes off the road
Can be easily mounted to handlebars on bike
Separate mobile docking stations can be purchased (great if you plan to use it in your car and bike)
Home docking kit available
There is a rubber case available, which will protect it from the elements
Price:
Main unit with mobile kit: $329 (rebate is available)
Home kit: $99
Extra mobile kit: $99
Rubber case: $15 (ebay)
The only thing I have to do to get it working on my bike is figure out a way to power it. It has a cigarette adapter, so I figure I'll need to install one on my bike. I will probably need some input and advice from the forum members on this, because I'm not the most knowledgeable person when it comes to electrical systems and doing this kind of thing.
If you want a versatile unit that has controls ideally suited for a motorcycle, this is the one to get.