1. Both series are tragic.
2. Both series have a lot of deceiving and mind screwing plot twists.
3. Both series involves bloodshed.
(Although Requiem for the Phantom has a more realistic feel while Kuroshitsuji II has a supernatural-fantasy feel.)
1. The first Kuroshitsuji is not a strong as the second one and is more focused on comedy. On the other hand, Kuroshitsuji II is darker, more serious, has more action, bloodshed and definitely more mind screwing plot twists. But still, you must see the first one before you could dive into the main course.
2. The same thing goes for Requiem of the Phantom. It started out pretty slow, although it is good, it is not as good as the next time shift and it gets better as it shifts further into the future.