] Is it only a difference in the mode of travel available, Mr Darcy could afford better horses to travel with, or am I missing something?

This exchange reveals more about Darcy than he knows. He is feeling Lizzy out with his provocative statements:

"It must be very agreeable to her to be settled within so easy a distance of her own family and friends."

"An easy distance, do you call it? It is nearly fifty miles."

"And what is fifty miles of good road? Little more than half a day's journey. Yes, I call it a very easy distance."

"I should never have considered the distance as one of the advantages of the match," cried Elizabeth. "I should never have said Mrs. Collins was settled near her family."

Darcy is in both discovery and selling mode here.
Wouldn’t she agree with him about a bride being possibly too close? Darcy, IMO, is so sure of himself, he is not really expecting her to disagree.

"It is a proof of your own attachment to Hertfordshire. Anything beyond the very neighbourhood of Longbourn, I suppose, would appear far."

As he spoke there was a sort of smile which Elizabeth fancied she understood;

Which she did not. Lizzy assuming the bride would be Jane, not herself.