With all the expertise of a new graduate of HR Block's tax prep course, I can tell you they're basically the same. Well, as of 2008. I haven't heard the '09 tax year updates yet. They serve the same function (help you save for retirement), have the same maximum contribution limits, and same age limits for making contributions or taking withdrawals.
This fairly short article sums up the similarities & differences.
AFA the rollover, you can roll between 401k and 403b since both are funded with pre-tax dollars. Just be sure to roll directly between them to avoid tax problems -- don't have it distributed to you first. I'm sure if you call Vanguard, they can handle it for you.