Most people know I'm dating Crompsin. I'd totally take him over my cat/horse on an island. Sure, my cat might be able to feed herself.. but she'd be a liability. Horses take a lot of effort to feed and a single injury can be devastating. Crompsin has great survival skills and I'm positive that if we got stranded he'd have tons of useful tools with him (as usual). We get along quite well, especially when we have work to do.. so building a shelter, finding food, cooking food, cleaning, etc. would be fine for both of us.
We were actually watching a (badly) dubbed version of Robinson Crusoe last night, so this thread is amusingly related.
I was actually extremely interested in Crusoe being stranded with only a dog to keep him company.. and talking to the dog was really his only way to hold onto his humanity. When the dog died, he was devastated and really became "wild." I'm sure that having a human companion would be more fulfilling than an animal (humans have imaginations, can tell stories, etc.), as well as generally more useful.. though it would be particularly painful if something were to happen to him/her.