Dogs require a lot of responsibility. They need walking, grooming, bathing, training, socialising, playing etc.. It is essential to train dogs, otherwise they'll become reckless. Some dogs suffer really badly on their own, and you cannot leave them on their own for long periods of time. Depending on the breed they will need plenty of exercise. They also need to be well socialised with other dogs because if not, then walking them will be a hard task if confronted by another dog.
Ferrets also require responsibility, but i wouldn't say as much as a dog. They need a large enclosure with loads of toys and enrichment. It's best to get a male, because if females come into heat and they're not mated to, they will become very ill and have a possibility to die. They too need plenty of exercise, you can get harnesses for them to walk them. Also you can train your ferret, but i don't think it's as essential as it is for dogs. Ferrets to have a tendency to smell pretty bad, but if you love animals like me (which im sure you do) it's easily tolerated.
I advise you to research them, joining forums that specialise in that animal and see which you think you'd prefer
If it was me, i'd choose a ferret. I think dogs are too much like forever children (literally) which i don't feel i'd be ready for at this moment in time. Haha