I played and beat dark souls first, and then I went to demons souls. Both brilliant games, but I think demons souls was a bit easier if I'm being honest. It was incredibly anti climatic at the end, which was disappointing. Dark souls at least has a good fight with Gwyn at the end of it to finish it off.
One thing I did like more about demons souls was having the nexus there as a home base. Dark souls has firelink shrine, but even then it doesn't really feel like a 'home'. Love the completely open world aspect of it though.
One thing that has to be said though, demons souls can be really easy if you choose to use the exploit which gives you infinite items pretty much. Levels you up as much as you want but the game is still fun. Did that on my second play through because I wanted to beat the game properly first.
I hope they don't do any bullshit boss fights like bed of chaos or the ceaseless discharge. Those were the 2 worst and most bullshit fights. Took basically no skill. One you just had to run around the map a few times, and with ceaseless discharge the only way to kill him was to have him follow you until he fell.
Then there was the mpossible fight with the red dragon in undead parish, but it was optional thankfully.