Neither of these phrases seems like an entirely accurate description of Winterbourne, but faced with a straight choice between the two, one would certainly be compelled to choose the first. This is a rather over-generous and one-sided description of his character, whereas the second is a completely inaccurate one.
A fair description of Winterbourne would have to mention that he is kind-hearted and well-meaning, but indecisive, cold, and "stiff." The final word is one Daisy herself uses to describe him to his face at least three times, referring to his extreme and old-fashioned notions of propriety, his refusal to dance, and to his rigid formality in conversation. All this might describe someone who from a slightly differing perspective would simply be "a pure gentleman with the best intentions." Certainly Winterbourne is a gentleman and his motives are not bad ones, but he is also a dull and lifeless character, of the type a contemporary reader might have labelled a "stick."
In introducing Winterbourne, James writes that "he was an extremely amiable fellow, and universally liked." He also writes what the reader is apt to forget, that he is twenty-seven years old. Winterbourne's very name suggests a precipitous rush towards old-age, and his words and actions continually make him seem far too old for Daisy.