Tsuzuki Asato is a shinigami: a guardian of death. Acting as an investigator in Juuouchou, the bureau in charge of ensuring that each soul passes through the cycle of life naturally, his job often involves dealing with those who would disrupt this cycle. His life after death is about to change, however, as he is assigned a new partner, Kurosaki Hisoka, for a case in Nagasaki - a case seeming to involve some sort of vampire. But the two shinigami soon find that something deeper lies behind this blood-sucking menace...
In the mystical world of Shangri-la, demons and humans live side by side, watched over by a parthenon of ancient Chinese gods. But when normally-civilized demons start to go berserk, the gods require the services of Genjou Sanzo - a Buddhist priest with a magical gun, an evil-banishing scripture and a take-no-prisoners attitude. Aided by the ancient monkey god Son Goku, the half-demon Sha Gojyo and the demon exterminator Cho Hakkai, he sets off on a treacherous journey westward, with armies of demons, dark mages and angry gods all standing in his way...
Descendants of Darkness and Saiyuki both have decent detective stories. Both are fairly predictable, but decent.
For those who enjoy boy-love Saiyuki I think has the better relationship, because the characters are less, well Descendants of Darkness gives the main characters a somewhat flat personality, and it can be a bit of a drawback at times. However, on the same token Descendants of Darkness also has a much more satisfying amount of fan-service (if, as seems to always happen, it does detract from the story a bit, which is a shame).
Saiyuki -is- significantly longer at 50 eps vs. only 13 though, which could be an issue for you if you don't like or you prefer longer or shorter stories as the case may be.
Ultimately though, these two anime are roughly on par with one-another for quality and very similar in content without being too similar.
Another thing to consider is that while Descendants of Darkness is an absolutely beautiful gothic and macabre world, Saiyuki is very much of a kind of. .. illustrious beauty. Alabaster pillars, colorful outfits and so forth permeate the majority of the show. Though by pure quantity and not percentage all of its short moments of the macabre may add up to just as much as is had by Descendants of Darkness heh heh.
Subaru Sumeragi is a descendant of an ancient clan who wields immense powers of the yin-yang. Because of his supernatural abilities, he doubles as an investigator for paranormal crimes and situations in Tokyo. With his twin sister Hokuto and comrade Seishirou Sakarazuka at his side, Subaru soon becomes involved in two cases which may cost him his life!
Apart from the shounen-ai overtones, Tokyo Babylon and Descendants of Darkness offer solid entertainment as far as supernatural thrillers go. Mixing spells with detective work, these anime share the same vibe.