What is similar and different between Kurt's and Manager's characters and why does Marlow remain loyal to one and not the other?
Similar is that both characters are initially an enigma to Marlow. However, Marlow soon figures out that while Kurtz has gone into the heart of his own soul or in other words the heart of darkness, the Manager has made no such journey. It seems to Marlow that the Manager is hollow inside - he calls him a "paper maiche Mephistopheles" and indicates that although both Kurtz and the Manager have been in the jungle for a long time, its effect on the Manager is to make him more hollow. That would be the major difference.
There is also an indication that while the Manager is not unintelligent; he does not inspire devotion or respect from the people he oversees. Kurtz is described as being great and inspiring respect from all different quarters.
As for similarities, they are both working for the company and are both corrupt and seek profit for their themselves. Additionally they both must have had the stamina to survive and to seek profit.
It is because of Kurtz's intelligence and Marlow recognising some of himself in Kurtz and Kurtz's inner journey that Marlow remains loyal to him. He does not remain loyal to the Manager as he sees him as an empty husk with no inner life.