The Excelsior was really an adversary ship, so they likely made it look more squat, wide and powerful from the front end to emphasize that. The Enterprise is a "tall ship", Excelsior is the Merrimac/C.S.S Virginia. With that big black grill on the engineering section it kind of reminds me of ED-209. From the side its a different story, long and elegant lines.
Anyway, I like the Challenger, nice take on the Refit styling.