The differences in the two roads are very subtle, as suggested in stanza two:
Then took the other, as just as fair,
And having perhaps the better claim,
Because it was grassy and wanted wear;
Though as for that the passing there
Had worn them really about the same,
'Just as fair' suggests that the two roads were equally appealing in physical appearance i.e. the one did not look more attractive than the other. The word 'just' suggests equality. The only reason the speaker chose the second road was that it had more grass growing on it than on the other, and thus had a "better claim." This gave the speaker more reason to choose it and provided the impetus for him to follow it. Maybe, because it had a more lush covering, it
could also make his journey somewhat more comfortable.
The end of the stanza also proposes that both roads seemed to have borne an equal amount of traffic. The one road did not seem a more popular choice than the other, for those who had previously taken them. Both paths had been
'worn ... really about the same'.
Another probable difference is intimated at the end of stanza one.
...long I stood
And looked down one as far as I could
To where it bent in the undergrowth;
It is clear that the speaker took a long, hard look down the first road to see where it lead. He could see only as far as the place where it disappeared into a bend. This ironically suggests that the speaker had already made up his mind to
take the second road, even though he claims that they were equal. It means that this particular road appeared less attractive to the speaker than the route he took. He hardly spent any time to scrutinize the second road which implies
that he was pleased with it from the outset.
Stanza three further emphasizes the fact that the roads were not different at all:
And both that morning equally lay
In leaves no step had trodden black.
It is clear that the two roads had seldom been used, for none of the two displayed leaves which had been blackened by feet repeatedly treading over them. In this respect they were also the same--the one had not been used
more than the other.