Jills more likely to be more nippy with people IMO, hobs more likely to kill underground due to more size and power.
Jills handier as they're smaller, so you can carry more of them around with you. I like to have a hob, but I always think they're a bit overkill for ferreting. If you only need jills to bolt rabbits I don't see the point in using big powerful hobs (in most circumstances).....however, if you've got a big warren, or a warren where rabbits are reluctant to bolt, a big hob will usually get there arses flying out of there a lot quicker than jills.
I've got quite a nice lit