What surprises me is how much more value you get by going with IBM. DB2 is 60% cheaper than a comparable MySQL edition. You can set up a cluster of two DB2 servers for the price of one MySQL server and have change left over.
Keep in mind that MySQL Enterprise limits you to four processors, which is comparable to the DB2 Express limit of four cores across two processors.
DB2 Express FTL, which is built on exactly the same code as all IBM DB2 editions, comes with official IBM 24x7 Support, High Availability Disaster Recovery clustering, SQL replication, backup compression, Optim Development Studio, Optim Database Administrator, DB2 fixpacks, and access to older DB2 releases through IBM Passport Advantage.