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Ubicom sx uses software virtual peripheral, it is a good MCU if your design is not time critical, and I think more design resouces are available for Atmel avr's and more people are using avr's, for learning, I think both of them are OK.
1.What kind of application you want to develop (however, SX is for time critical design - it has 75 Mips with nice features you wont find on AVR)
2.How would you like to learn:
- low level assembler (much easy for SX if you're coming from PIC world)
- high level C (AVR is much suitable for C than SX, beyond the portability of C, with great support from the field)
3.Future availability and development of microcontroller (SX is dead on development if we're not looking at IP2022)
4.Technical support if you're in trouble (great for AVR, much more debate forums than SX)
5.Price (it's up to you)
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)