Except that once you start the upgrade process and get to a verification error, which the OP did before he ever even posted, you often can't go back to the firmware you were previously on, right? Unless, of course, you have blobs for that firmware. We get plenty of people posting here with that issue, and the advice is always the same--you're stuck with current firmware--so I assumed that was the case here. Anyone please do correct me if I'm wrong.
When I upgraded, I actually chose to go to 4.2.1 over 4.3.3, as 4.3.3 users were reporting too many overall issues and specifically problems with preserved basebands. If you can get to it, the 4.2.1 redsn0w JB is nice and stable. The only thing I'd like from 4.3.3 is the ability to redownload music from iTunes. Other than that, there's not much difference.