This is a great question! One of the central differences to me is how they are described and depicted. One of the first things that strikes Marlow when he sees the Russian is his resemblance to a harlequin:
He looked like a harlequin. His clothes had been made of some stuff that was brown holland probably, but it was covered with patches all over, with bright patches, blue, red, and yellow - patches on the back, patches on the front, patches on elbows, on knees; coloured binding around his jacket, scarlet edging at the bottom of his trousers; and the sunshine made him loom extremely gay and wonderfully neat withal, because you could see how beautiful all this patching had been done.
The photojournalist in the film is dressed normally in drab clothes. There is also another difference in their youth and innocence. The harlequin is given a "beardless, boyish face" and reference is made to his "little blue eyes that were perfectly round." The photojournalist, on the other hand, is presented as someone who is anything but innocent - he is almost presented as someone on drugs and as someone who has definitely seen the world.
Where they are the same, however, is in the way that they have been dominated totally by Kurtz and his charisma. Both would do anything for him and are in awe of Kurtz.