Basketball Reference has historical AP poll data from the 1948-49 season on. Let’s take a look! It looks like the final AP poll was conducted BEFORE the tournaments until 53
1949
NCAA champion Kentucky was ranked number one in the final AP poll. Number 2 Oklahoma St and 4 Illinois were also NCAA participants. NIT champ San Francisco was 8. Kentucky also played in the NIT, losing to number 16 Loyola. I believe this was the Ralph Beard point shaving game. Number 3 St Louis, 5 Wesrern Kentucky and 7 Bradley were also in the NIT. Since NCAA champ Kentucky lost in the NIT, it is probably fair to call San Francisco the rightful national champs, though the point shaving complicates things and I think it is for to say the tournaments were fairly even that year
1950
CCNY beat Bradley in the title game of the NCAA and NIT. Again the final poll was apparently before the tournaments as Bradley was rated first and CCNY 19. The NCAA tournament also featured 2 Ohio St, 4 Holy Cross, 5 NC State, 7 UCLA. The NIT included 3 Kentucky, 6 Duquesne, 8 Western Kentucky, 9 St John’s and 10 LaSalle. Pretty even.
1951
NCAA champion Kentucky finished the season number one. NIT champ BYU finished 11. The NCAA tournament included included 2 Oklahoma State, 3 Columbia, 4 Kansas St, 5 Illinois, 8 NC St, 9 St John’s and NIT champ BYU. The NIT also had 8 NC State, 9 St John’s and 10 St Louis. 7 Indiana did not play on either tournament as Illinois won the Big Ten and the NCAA bid. Clearly the NCAA was the superior tournament this year.
1952
Once again our final AP poll appears before the tournaments. NCAA champ Kansas is 8 while NIT champ LaSalle is not rated. The NCAA tournament also featured 1 Kentucky, 2 Illinois, 4 Duquesne, 5 St Louis and 10 St John’s. The NIT included 4 Duquesne, 5 St Louis and 10 St John’s. The NIT is closer in quality this year than 51 but all the top ten teams in the NIT were also in the NCAA tournament.
1953 has already been discussed in detail, with the NCAA being the better tournament that year as well. I think we can safely say that in 49 and 50 the two tournaments were extremely close in quality but the NCAA gained an advantage by 51