Marlow and Hastings are both "unsuitable suitors" but in different ways. Marlow is the more honest of the two but incurably bashful and awkward around women. His good looks and scolarly bent don't tempt Kate because she considers he has the makings of a jealous husband.
Hastings, the more jovial and colourful character of the two, has more posh and charm but lacks the moral virtues of his friend. He is willing to resort to deceit if it means he can get his lady.
For a more detailed profile of these and other characters in this play, check out the reference below.