It's the lifetime of the specific radio.
You can transfer to another radio up to 3 times, I believe, for a 75 transfer fee each time.
And the lifetime offer ends, well, tomorrow.
XM also offers multiyear discounts, with no cancellation penalty; Sirius's multiyear discount plans have a 75$ cancellation fee.
Big differences:
XM: I think they have geosychronous sats.
Sirius: sats move [I think]
XM: Opie and Anthony
Sirius: Howard Stern next year
XM: MLB, Nascar, golf
S: NFL
XM: Deeper playlists, more stuff you havent' heard in a while, or never heard
S: More songs you may be familiar with, more repetition
Monthly subscription fees are equivalent. XM has Family Plan, each add'l radio up to 4 can be added to an account for 6.99. Not sure if S has that.
Hardware: I think S has more hardware available; but XM is catching up.
Automaker support: XM and S both have specific automakers installing sat-rad-ready devices in some of their new cars with a free trial sub; I'm not sure what carmaker is with whom.
They both have the online service included in the monthly fee. XM offers a few channels that are only on the online service [Special X, On the Rocks, XMLiquidMetal, The Eye], and some that are only on the birds [most of the 'network' talk/news channels like FNC ABC etc], and that is due to licensing issues. S is probably the same, but I don't know for sure.
We've had XM for probably two years now, and love it. Every day I hear something I like that I've never heard before, or I hear a comedian I like on the comedy channels. I was a little unhappy when they raised the price from 9.99/mo to 12.99, but they included online access in that, which helped a little.
I get a good signal except:
* in my garage
* under my parking deck at work
* for about 50 yards driving down this road that is totally surrounded by really high trees
If you like the radio, or miss what radio used to be, you've got to give satrad a chance. It's lightyears ahead of most terrestrial radio.
I personally don't care about 'extreme radio' [Stern or O&A] or sports, so those two were non-issues for me. I wanted deep, wide playlists with a large variety of music, and XM, imho, has the better music playlists.
Favorite channels:
Fungus [punk/ska], Boneyard [rock/metal], Fred [classic alternative], Cinemagic [movie scores and clips], Comedy [uncensored comedy], On the Rocks [lounge music], Special X [assorted weirdness, Fox News Channel