Martins show was a huge success. He had a hit stand up in 93 also hosting def Comedy Jam which was also huge at the time. Martin was a big name.
I think a lot of people always think of Will as a big star but he wasn't yet. He was still growing into who he would become.
Plus Martin was a creator and writer on the "Martin" show where as Will did not create or write "The Fresh Prince" show though it was based around his Rap Persona (Quince Jones was a creator). Will was a Star at that point but he wasn't a Mega/Super Star yet until after the "Bad Boys" through "Enemy of the State" run. So yes Martin was a Big Name at that point which is why he got the Staring role originally in "Bad Boys" and had to bring Will on to the movie.
Would have been the perfect run if he picked The Matrix over Wild Wild West in 99.
Yes huge misstep on Will's Part. "Wild Wild West" was a huge show from the 60's, he should of known taking over a beloved role like that would of been iffy. The Movie actually still did over 200 Million Worldwide but the budget was crazy something like 170 and that was huge for a movie back then so Will got paid on that. Will did get a #1 Hit Single from the "Wild Wild West" song out of it though, lol. Everything happens for reasons sometimes, Keanu was amazing as Neo but yes the "One" being a Black Man should still happen. 99-01 wasn't the greatest run for Will outside of "Ali" though sadly that didn't do to hot at the box office.
99 - Wild Wild West
00 - Legend of Bagger Vance
01 - Ali
His most successful Run came right after that slump in 02-08 where he made history having 8 straight 100 Million Dollar Blockbusters in a row:
02 Men Black II
03 Bad Boys II
04 I, Robot and Shark Tale
05 Hitch
06 Pursuit of Happyness
07 I Am Legend
08 Hancock
These last 10 years have been hit or miss honestly but he's been on the right track this last year.