Mehdi,

I think you'll find the AVR a capable microcontroller. I have very little to criticise about it. They have an active forum called www.avrfreaks.net. I've used the Imagecraft ICCAVR compiler with great success and can recommend it for the price (~$199usd). Don't go to avrfreaks and ask "which compiler is best?" as a million people before have asked the same question, you can search on this topic and see the results! A couple of lessons I learn quickly are:

Remember that many instructions only work on R16..31
Remember that it is a RISC architecture - you can only read or write to memory.
The number of registers can become a curse - I used the register rules for the 'c' compiler (imagecraft) when writing assembler, this saved me many headaches.

Apart from the above, it's all good (most of the time!). After some experience, you'll probably come to appreciate the 8051 a little more.

Do yourself a favour and use 'c' where possible - this may limit your reliance on a particular architecture.