We have both from when we lived on the farm.
If it was just me, probably just a small dog.  Not like the ones we have now.  (Lab/German Shepard cross and Husky/German Shepard cross)
The cat never really causes any real trouble.  Let it out in the morning, let it back in at night.  I don't think it does anything in the house besides sleep. 
For 14 years on the farm we had a Pomeranian.  That dog was a trip, whipped the 2 bigger dogs everytime when they were pups.  He was king of the house.
A Sheltie could be fun.  And if I decide to ever get sheep, they're damn useful.
Just anything smaller than the cattle we have now.. 
