Both are about a disturbed, deranged young woman who stalks and harasses the male protagonist after a chance encounter and doesn't hesitate to harm those close to him to get to him. The main differences are that Zashiki Onna starts off fairly mild and builds up tension and terror as it goes along whereas Ibitsu goes full tilt from the beginning, and that the stalker in Ibitsu is developed and explored a bit more thoroughly than in Zashiki Onna. Other than those factors, they're pretty much different takes on the same story.
Those, both rather short, horror mangas contains an extremely frightening and sick woman who is stalking the male lead.The mangas are both very thrilling and if you want something short and creepy, I'd recommend them both. If you liked one of them, I'm pretty sure that you'd like the other one as well.
Both have creepy women that are relentlessly stalking the male protagonist and those around him. I felt that Ibitsu was creepier. If you are not a fan of gore in manga, I suggest Zashiki Onna rather than Ibitsu. If you don't mind a bit of gore, then Ibitsu is probably superior.
If you enjoyed Ibitsu, you will enjoy Zashiki Onna and visa versa.