On the whole, it's fair to say that the differences between Ahab and Ishmael greatly outnumber the similarities they share.
Ishmael is a highly-educated young man, open-minded about the many experiences that life has to offer. By contrast, Ahab is a man with a narrow, one-dimensional outlook on life: a deranged old sea-dog obsessed with hunting down and killing the great white whale that bit off his leg on a previous voyage.
To be sure, Ishmael is fascinated by Moby Dick, but unlike Ahab, he isn't obsessed with it. The main reason for this, of course, is that Ahab has a very personal connection to the whale. If Ishmael's leg had been bitten off by this fearsome sea beast, there's every reason to believe that he would've felt the same way as Ahab.
As it is, the main similarity that exists between these two fundamentally dissimilar men is that they are, to a considerable extent, estranged from society. Bored by life in New York, Ishmael takes to sea in search of adventure, fresh air, and exercise. A long and potentially hazardous journey aboard the Pequod is just what he needs. In stepping aboard the whaling ship, Ishmael is turning his back on a society in which he feels he doesn't truly belong.
Ahab doesn't really belong in society, either. The sea is Ahab's natural habitat no less than the great white whale's. The captain is destined to die at sea, whether by drowning or being sliced to pieces by the whale's fearsome jaws. Either way, it's hard to imagine an old sea-dog like Ahab dying peacefully in his sleep on dry land.