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).
Bentley Drummle is a member of the minor aristocracy from Somersetshire: "the next heir but one to a baronetcy," and is "an old-looking young man;" whereas Startop in contrast is "younger in years and appearance." Ch.23.
Moreover, Bentley Drummle is a "sulky" fellow who "was idle,proud,niggardly,reserved, and suspicious. Startop on the contrary "had a woman's delicacy of feature, and was...-exactly like his mother." Ch.25.
Both of them are his mates in Mr.Matthew Pocket's 'tuition academy.' Needless to say Pip was attracted to Startop who later on in Chs.52-4 plays a very useful role in assisting Pip to help Magwitch escape from London.
On the contrary, Pip detested Drummle. He had contempt for Drummle because Drummle belonged to the idle rich aristocracy. For Pip, Drummle was proof enough of the arbitrary nature of class distinctions: social advancement and prosperity had no connection to moral worth or intelligence leave alone hard work and self reliance.
Furthermore, when Estella is determined to marry Bentley Drummle Pip does his best to prevent her from doing so by describing him "as ungainly within , as without. A deficient, ill-tempered,lowering stupid fellow....[with] nothing to recommend him but money, and a ridiculous roll of addle-headed predecessors." Ch.38. Clearly emphasising the fact that he is an unromantic lout unlike himself who passionately adores her.