Re:Monster, Tensei shitara Slime Datta Ken Recommendations
Both main characters have similar abilities; when they consume material(Monsters, ores) they get some of the materials ability's. Ex. magician, they would get an ability of the magician or the class(RE:monster)
They both start ruling over monsters, making they strong, builing a base, and overall expanding they colony from a small one to a big one.
Basically they are the same thing but in a different style Re:monster is in a diary style and Tensei shitara Slime Datta Ken is POV.