I knew nothing about SLRs or external flash units prior to buying the D60 and 550ex. I know have 2 420s along with my 550. From a learning standpoint, the 550 acts as a great teaching tool. The manual features allow you to control so much more than the 420 has to offer. Of course, you could leave it on full auto, or do some (or all) manually.
The added benefit of using the 550 as a master and the 420 as a slave will bring so much more to your photography.
I would say get the 550 and keep the 420 as a slave. You won't be sorry.