Depends on the mission role.
But the one thing we know about the Excelsior
is that she was supposed to be fast. The Constellation
couldn't be an "insurance" design for Starfleet's great experiment at fastness unless she were designed to be fast, too.
Absolutely, I agree. I don't think the Constellation was a back-up, I think it had a different role. A deep space explorer, capable of operating over long distances and being relatively self-sufficient.
The Excelsior is a different kettle of fish; an experimental, breakthrough drive system built for speed and power. I do see her as a replacement for the kind of jobs we saw from the Enterprise in TOS and later TNG: police actions, ferrying diplomats, hosting conferences, and sometimes exploring the galaxy.