I would say go for Demon's first. The hub nature is good cause you can always go back and try a different world if you're stuck, and you can carry a shit ton of healing items,if needed. Not saying this in an offending way, mind you! I'm just saying that it's probably better to start from the game where it all started, as Dark Souls' builds upon that foundation. Some things are easier (practically no limit for healing items or magic uses, hub world makes it easier to explore without getting lost) while some are harder (weight limit, some bosses are nasty, world 5 is worse than blighttown imo).
Some bosses are a bit hard for new players (more than Dark Souls, imo) but you'll be fine if you are a patient person. The game isn't "hard"..it just punishes impatience and recklessness. If you keep an eye out for trouble, don't try to fight 3 enemies at once and don't heal like crazy in the middle of a fight (make sure you have the time to do it first) you'll be fine..