Since I’ve been following both Salman Khan and Shahrukh Khan closely, I believe I am good candidate to answer this question.
Salman and SRK’s career trajectories have always been gone through ups downs. It has directly impacted on their fan following both positively and negatively. I will analyse their fan following as per different timelines of their career:
1989–1991
When Salman debuted as lead actor through Main Pyar Kiya, he became overnight sensation. It was probably first instance when debutant gave all time blockbuster movie. Due to charming chocolate boy looks, Salman grabbed plenty of female fans. He capitalized on MPK’s success and gave back to back 5 hit movies (Baagi, Sanam Bewafa, Saajan, Patthar Ke Phool and Kurbaan). Meanwhile, Srk hadn't debuted in the movies yet. However, he acted in TV serials like Fauzi and Circus during this time and had gained immense popularity in family audiences.
Overall, Salman Khan was more popular and had more fans during this period.
1992–1994
Salman Khan gave back to back flops in 1992–93 which affected his career badly. Meanwhile, SRK debuted in movies through Deewana movie which became major blockbuster in 1992. Srk was praised for his acting and role. In 1993, he gave back to back 2 blockbuster with Darr and Baajigar. He was highly praised for his negative roles and strong performances which led him to win Filmfare Best Actor Award for Baajigar. At this point, SRK had garned quite popularity however from fan following perspective, Salman was still leading. He reinforced his fan following with Hum Apke Hain Kaun- biggest blockbuster of 90s decade.
1995–2009
In 1995, SRK had major blockbuster with the iconic movie DDLJ. This movie changed fan following picture completely. Movie became quite popular in Indian diaspora audiences. After this movie, Srk established himself as romantic chocolate boy and garned pretty much female fan following. He also became popular in family audiences. He continued his romantic boy legacy with blockbusters like DTPH, KKHH, Mohabbatein, KHNH, Veer Zaara,Om Shanti Om and Jab tak hai jaan. Along with this, he gained significant acting praise through movies such as Devdas, Swades, Chak De India and My name Khan etc. On the other hand, Salman had stable career throughout 90s. He was also capitalising on his romantic and charming boy image to deliver major hits like Jeet, Pyar Kiya to Darna kya, HDDCS, HSSH etc. He also did comedy movies such as Judwa and biwi no 1, action movies like Karan Arjun to grab family audiences and female audiences as fans. 2000s decade was not very good for him. While he gave few hits at starting of the decade, he gave many flops until 2010. During this time he delivered movies like Tere Naam which established his image as macho sigma male and contributed a lot in gathering young male audiences.
At this period, SRK had more fans. His Indian diaspora, female and family audience fans overweight salman khan's male and family audience fans.
2009–2022
During this period, Salman was on his peak of his career. He Gathered a lot of male fans through movies Wanted, Dabangg, Bodyguard, ETT, Kick, TZH. Meanwhile, he also garned a lot of family audience as fans through movies such as Bajrangi bhaijaan, Sultan, Ready. Through this movies, he also got loyal fan following in middle East. Meanwhile SRK was doing good until 2013. Post Chennai express, his movies started to fail and it impacted his fan following badly. His Indian diaspora fans and metro fans were still intact however Salman Khan fans completely overweight them in numbers. During this Salman had delivered three 3 cr footfalls movies(BB, Sultan and TZH) which shows his huge fan following.
From 2023:
Post disastrous performance of zero in 2018, SRK took almost 4 years break and did a come back with Pathaan. His fans who were awaiting keenly for his movies, flocked theatres. Also, Cameo of Salman created a lot of buzz and his fans also came back to theatres to watch Pathaan. Due to collective fan efforts, Pathaan became bollywood’s first 500 cr Indian net collection movie. After this, SRK made Jawan- another mass comercial movie and got bigger success than Pathaan. He became first bollywood actor to deliver two 1 thousand crores worldwide grossing movies. He also came with Dunki in same year which had created a lot buzz since he was working with Hirani for first time, however movies received mix reviews and become average grosser. On the other hand, Salman Khan career went in downward direction. He delivered multiple bad and flop movies. Movies like Dabangg 3, Radhe, KKBKKJ and sikander destroyed his legacy badly and impacted his fan following in tier 1 and tier 2 negatively. Meanwhile movies like tiger 3 who had generated high buzz, turned out to be average at box office.
During this period, Srk grabbed a lot of tier 2 and tier 3 fans which were fortress of Salman once. He also kept his Indian diaspora and metro fans intact. Due to this, at the current point, SRK has more fans than Salman. However, table can turn any time in the future.