I thought the 8051 was dead years ago, but like Lazarrus it came back from almost certain death much stronger and more diverse.

Any experience with any microcontroller is going to advance your skills regardless of the exact type. I apply the same knowlege and techniques for a 8051 as I would for an ARM, AVR,68K whatever. So don't worry about betting on a losing horse! With the amount of new product using 8051 architecture released in the last 5 years, I dare say its going to be with us for a few more years.

With Microchip, their new PIC24 series is architecturely miles away from the PIC12/14/16/18 series. So one might think the PIC users are going to have to change a little in the future.

If you do your code in 'c' - moving from one micro to another is a lot less painless than rewriting reams of assembler code.

My advice - stick with one type first up, get some experience then look at other devices. With some experience, you'll be better equipped to understand the differences and adapt your knowlege to suit.