"its an Investment" that you are going to bless your self in future for doing so,so get anyone, It's not necessary that you start with a uno/or deumilanove (deu is obsolete, but i love them) you can also start up with a mega , the learning goes in the same way no special ways you have to work.
Advantages of a Smaller UNO/duemilanove like board:
1) its small not many pin's but enough for a beginner.
2) cost is too less so if you fry it its fine not that much cost to incur to have a new chip.
Advantages of a Mega as starter:
1) The biggest one available.more than 50 GPIO's.
2) cost is high but then you get a whole lot of power like EEPROM etc
The disadvantage of Mega is that it has a SMD uC on it so once you fry it most probably you have to get a new Mega.
The small boards have no dis advantage leaving their limitations.
Best of Arduino!