As impressive as the F5 was, many pro's who bought the F90x (N90s) chose to stick with it when the F5 was released.
My 5 and my 90
I would also have chosen the $1000 F90 in 1996, the F5 costed $3000,
which is $4,975.70 in 2020 dollars, using the currency calculator. Mine costed $250 from japanese ebay seller, like new. Right now price has increased a bit.
Then add that by 1997 Pros were starting to realize that future was to be digital, a reason to remain with the F90 and not throwing equivalent $5k in a F5, this was a long term investment in a moment when technology was to change radically:
Kodak DCS 760 was a Nikon F5 with a Kodak 6Mpix digital back, and the Kodak DCS 420, was a 1.2-megapixel digital SLR based on a F90 body.
This is the main reason why some Pros chose to stick with the F90, they wanted to see what was to happen before throwing today's $5k.
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Regarding the 5 vs 90...
I love the 90 and I use it from time to time, but belive me,
the 5 is another world. As mentioned, to not go to Canon, many Pros moved from the F4 to the F90 which had a better AF, the F90 is an advanced prosumer that can could be used well proffesionally, but the F5 plays in another league.
First a Pro valued of the F5 is its Matrix meter. It does not fail, never. You put more or less sky in the framing and matricial exposure stands stable because the camera interpreted the scene correctly, the thing does not doubt. 1005 RGB points covering all the frame. The camera intelligence (Neural Network) recognizes sky, water, snow, vegetation, and human skin. This was an impressive achivement in 1996 ! Imported from future.
The F5 also takes the latitude class from the DX and feeds that in the Neural Network, so it exposes perfectly CN, BW and slides. A Pro may rely in the F5 Auto meter to concentrate attention in a moving subject and in its expression, the camera won't fail and in average it will expose better than manually an skilled guy behind the F5.
A Pro had more reasons to prefer the F5... The F5 AF sported such a violence that cracked the gearings in some AF discount glass...
Finally, the TTL flash control of the F5 rocks, if fails not a single frame. Well, not finally, there are many other things...
Say we are wedding photographers... of course even the slow hassie may make an atonishing job in a wedding, but the F5 has the capability to get in the middle a group of people talking and to get several shots with natural expression before somebody noticed the photographer was there, it's ultra fast and precise, and every avaliable light or flash exposure is on spot...
For example, in this dynamic shot
https://www.flickr.com/photos/125592977@N05/21478354193/ the kid was moving a lot, but I could nail focus exaclty where I wanted:
Fortunately later the expression was sound... I could not play total attention to all... focus, exposure, expression, composition. Is in those situations that a F5 shines like the sun, once you are used to this camera you team with it.
Of course if you are to photograph a mountain on tripod (or a mostly static subject) then a FM will do the same, provide one knows how to meter,
So (IMO) the F5 advantge is prominent when shooting is dynamic, if not what remains is the armor and the sealing. One may not do dynamic shootings, one not may shot in the rain, one may not go to a war to shot film. One may not need a F5 at all...