Cats shed, that's for sure! So even though they are better at being left alone, maybe a dog would be better. Some breeds of dogs do not, poodles, maltese and some other dogs have hair that grows, not fir that sheds. They therefore need to be groomed, but you can learn to do it yourself.

An adult dog would be better than a puppy at being left alone for hours a day - are we talking 3 hours at a time, or 8-10 hours?

Every animal is different, pehaps you should play with a cat and play with a dog and compare which you liked better!