What is the difference in Nozzle 1.75mm and 3mm in maker bot?

Hi,

I am interested to buy Makerbot RP machine. But in the website they mention the nozzle as 1.75mm. With this 1.75mm can i get parts smoothness or not?
In 3d stuffmaker i have seen the option for 3mm nozzle. can i get differenece which size nozzle can i prefer.
waiting for reply.