Rats are very, very, very different pets to mice... aside from the obvious fact that you will need MUCH more space to keep and breed them, they also require MUCH more attention than mice do.
Also, it is recommended that you do not breed rats young, 5 months old at the youngest for the females I have been told. And I have also been told they are much more prone to hard labours (babies getting stuck etc) I personally have never had this, but I have only ever bred very few rat litters.
As to your question, rats gestation is slightly longer than mice *sometimes* depending on the size of the litter... mice pregnancy ranges from 18 to 21 days, whereas rat pregnancies range from 21 to 25 days.
Thanks WillowDragon, I think i'm going to opt for mice. However I have tried all the pet stores around here and they ae pretty much impossible to find, I was wondering if there was any other way I could get mice (I'm not sure if they can be posted? By a special courier maybe?)
Telling people where you are may help get mice to you! Although, if you have real trouble maybe you could look into breeding dwarf hamsters or something for food instead?
I was a rat breeder before I was a mouse breeder, and the amount of work and space involved with rats is INSANE. I have more mouse cages than I had rat cages and it's about a fifth of the work to clean the mice out. Plus, rats are highly intelligent and need interaction to be happy, whereas mice don't really mind either way.