Hi marmelado,
My advice: Buy a development board according to your budget.
Get used with either C or assembler. You can switch later to other family micro if you feel that need (easier if you start with C).
Mean time take a look at micro architecture Harvard and Van_Neuman, CISC or RISC. It's plenty of information over internet.
Finnaly, don't mix apples with oranges. (8051 and AVR, not necessay in that order).
Though 8051 it's my lovely micro (and that's I'm currently using), I must admit the strength of AVR architecture that was developed in conjuction with C language expert (what I like most is 32 working registers and XYZ indirect address register)
Regards,
Silvio