I definitely think they both competed for the "5th Major." Miami is rather the Latin American Major, too. I think things might have changed when Acapulco moved from clay to HC. We'll see how it goes, but I don't see it as a diminished 1000 right now.
Perhaps it's more that IW has grown by leaps and bounds.
As I mentioned before, IW was in a fairly precarious financial position for a few years, heavily mortgaged, and there was even the threat that with new buyers the event might be moved to a different location. Thank you Larry Ellison.
IW has influenced tennis venues in a big way since 2009 when Larry Ellison bought it lock, stock and barrel:
The change in 13 years has been pretty startling...
The billionaire tech mogul purchased the event 10 years ago. Here's what he's done in that time
www.desertsun.com
Things may change in the future, but for now, IMO in facilities for both public and the players, IW Paribas stands shoulders above every other Masters in the ATP calendar year. The players have named it their favorite tournament for x number of years running. 475K attended in 2019, only surpassed by the 4 Majors, and it has the largest permanent tennis exclusive stadium (including the Majors) outside of the USO, as well as a permanent 8 thousand seating capacity Stadium 2.
Before that, it was clearly the younger child compared to the Miami Masters as far as attendance, etc. (The old Key Biscayne location had a lot of charm but it limited expansion) Miami yes, it always has had a latin vibe to it. However IW really does have quite an international flair to it too these days too.