In the part of the world where I fly (southern US), ADS-B is comprehensive. Check out the current coverage on this page (when it finishes loading, click the "ADS-B" button): http://www.faa.gov/nextgen/flashmap/ . You'll find that your route would have been well-covered, until you left the Florida coast.
Since XM is subscription-based, and ADS-B is "free", the primary tradeoff is full coverage plus some additional weather products (more like what you have available pre-flight, I think) on XM, vs. no ongoing costs for ADS-B.
It is a little more expensive to "get started" with ADS-B, since the receivers are still kinda pricey. But they're not as pricey as they were, and the ADS-B coverage is better than it was when I got into it. I've been very pleased with my ADS-B weather on the iFly.
That said, I've never had XM and so don't have personal experience to compare with.