Well.. think of it this way.

The rabbit is going to eat more. A lot more.
The rabbit will take more space.. a lot more.
The rabbit will make more mess.. you get the picture.
The rabbit will make less food (babies)
The rabbit will take more time

Once you get several rats, you can breed your own breeding stock, so you won't have to buy them very often (only ocassionally to avoid inbreeding)