I'm starting a job programming a 8051 microcontroller

So where is the doubt ?

1. Start with assembly language and get to grips the architecture of the MCU that you propose to use.

2. Then if you already know C and have coded for desktop applications then get hold of books/tutorials to learn how C is applied to embedded controllers. www.keil.com is a good place to hunt for these.

3. If you are new to C also, then step 2 can wait that much longer till you are very familiar with assembly and the controller architecture.

And finally - these are MY views and may not be the only way - it is better that sooner or later you find which is comfortable to you.

Raghu