A good way to start might be to compare the physical appearances of the two men. Drummle is "heavy in figure (and) movement", and has a sluggish look to his face. He has a "large, awkward tongue that seem(s) to loll about in his mouth as he himself loll(s) about in a room". In contrast, Startop has "a woman's delicacy of feature"; he looks a lot like his mother.
You might also want to compare the two men's backgrounds. Drummle comes from a rich family in Somernetshire, by whom he has been nurtured until being sent to work for Mr. Pocket. Startop has been raised and "spoiled by a weak mother, and kept at home when he ought to have been at school". Understandably, he is very much devoted to his mother.
The differences between the two men's personalities would also be important to note. Drummle is described as being "idle, proud, niggardly, reserved, and suspicious", a generally unappealing character. Startop is considerably more light-hearted and likeable, and Pip admits that he naturally takes to Startop "much more kindly than to Drummle (Chapter 25).