The two outlets are identical, with the exception of capacity. Both options have the same replication and SLA.
EDIT April 3, 2014 - Updated to reflect 500 GB SQL database size limit
EDIT June 17, 2013. Since I originally posted this answer, some things have changed with pricing (but size remains the only difference between web and business publications)
Web Edition scales to 5 GB, while Business Edition scales to 500 GB. Also: with the new MSDN plans (announced at TechEd 2013, see the ScottGu article in the blog post for more details), you will receive monthly cash loans for any services for which you want to apply your loans, including the SQL database (up to 150 US dollars per month, depending on the MSDN level), see this page for new MSDN benefits).
Both allow you to set a maximum size, and both of them are paid according to the amortized schedule, where your capacity is estimated daily. Detailed pricing details here . You will see that the base price starts at $ 4.995 (up to 100 MB), then goes on to $ 9.99 (up to 1 GB), and then starts a multi-level pricing for additional GB.
Regardless of the release, you have the same feature set - all about bandwidth limitations. You can easily change the maximum capacity or even change the version using T-SQL. For example, you can start with a web edition:
CREATE DATABASE Test (EDITION='WEB', MAXSIZE=1GB)
Your needs are growing, so you get up to 5 GB:
ALTER DATABASE Test MODIFY (EDITION='WEB', MAXSIZE=5GB)
Now you need even more capacity, so you need to go to one of the Business Edition levels:
ALTER DATABASE Test MODIFY (EDITION='BUSINESS', MAXSIZE=10GB)
If you ever need to reduce the size of the database, this works fine too - just change the permissions on the web edition:
ALTER DATABASE Test MODIFY (EDITION='WEB', MAXSIZE=5GB)