It's really a very, very minor difference unless you're playing euphonium-heavy grade 6 literature, or you have all-state level euphonium players.

It plays better in tune in lower register notes (below the staff to fourth line F) and you have access to more alternate fingerings, but no one ever uses those alternates anyway.

If you're spending the $9k for a good compensating euphonium, you may be better off buying another tuba instead.

And I say this as a euphonium doubler. I own one (a pretty decent Besson) - but that's because I bought it off a buddy who upgraded to a Willson for his primary horn.