I am an owner...errrmmm...slave of both! Piggies and rats are great in their own ways,but I am definatly a RAT person!!!
However the cons are rats live less then piggies in general and also female rats are prone to tumors,while RI's are common in all rats.I currently have 3 boys (I love the males).They are awesome and so sweet.
Rats are VERY smart and need AT LEAST 1 hour a day out of their cage.Males are also much more laid back then females.I love females too,but tend to like the boys abit more.They are more cuddly and more content to sit with you while you watch TV or go online,then the girlies who wanna run and play and explore.But then again,there are always exceptions...I have had hyper males and laid back females.But for the most part - if you want a lap rat,get males...if you want really playful,adventerous rats,get females.
Vet bills CAN be costly...depending.Surgeries can be anywhere from $100-$230 or more.
Do not use pine or cedar or any other sawdust/softwoods.Oh and rats LOVE hammocks.I find it gets pretty expensive if you keep buying hammocks when they wear out - so I make homemade ones out of old colthes,especially when my 4 yr old daughter grows out of hers!
I love my piggy too...but he is more into food and see's me more of his food supplier then friend,while my rats LOVE me and are constantly wanting my attention and are very affectionate towards me.
Good luck!