First let me tell you some intersting similarities in their career.
Amitabh started his career with an art film Saat Hindustani which won National awards. SRK started his career with Tv artsy TV movies like In Which Annie Gives it Those Ones and Idiot which won National Awards.
Amitabh got his first hit in a supporting role for which he got a filmfare award- Anand (1971)
SRK got his first hit in a supporting role for which he got a filmfare award- Deewana (1992)
Amitabh got his first Blockbuster in a serious film which won a Filmfare for Best screenplay and which got him his first filmfare nomination for Best Actor - Zanjeer (1973). The same year he got another filmfare nomination and won the award.(Namak Haraam)
SRK got his first Blockbuster in a serious film which won a Filmfare for Best Screenplay and which got him his first nomination for Best Actor - Baazigar (1993, unlike Amitabh he won it). He was nominated and won another filmfare the same year (Kabhi Haan Kabhi Naa).
2 years after his first Blockbuster Amitabh got two major Blockbusters one as a two hero film (Sholay). Sholay became one of the bigst actor gest hits of all time and Deewar also became a massive blockbuster. He has a moderate hit Chupst Actor ke Chupke.
2 years after his first Blockbuster SRK delivered two major Blockbusters one as a two herso film (Karan Arjun). DDLJ became one of the biggest hits of all time and Karan Arjun also became a massive Blockbuster. He has a moderate hit Ram Jaane
2 years later he got a major Blockbuster Amar Akbar Anthony. He followed it with another blockbuster Muqaddar ka Sikandar and Don and Trishul. He won filmfare awards for best actor both the years.
Similarly SRK got a major Blockbuster Dil to Pagal hai and some hits Pardes and Yes Boss and eventually have another big Blockbuster Kuch Kuch hota hai. He won filmfares for two consecutive years.
These similarities can go on and on. The point is both these actors have immense popularity as they both changed the way cinema was made in a major way. Amitabh Bachchan brought the era of Angry Young man where anger, class conflict, violence was supreme and music became less important. SRK on the other hand broke the the trope of violence, r**e scenes and introduced family films with romance and no villains.
Amitabh was a mass hero, SRK was more of multiplex, urban class herman took over. o. Amitabh was copied extensively by South stars like Rajnikanth and amassed pan-Indlean ian following. SRK became popular among the youth and the NRI audience.lean phase, was a phenomenon in countries like Ethiopia and Algeria, and SRK was instrumental in reviving the overseas market of Indian films. His overseas popularity is next to no one even now. Amitabh became the number one star in 1975 and constantly remained number 1 till 1991 after which he was replaced by the new generation. SRK became number 1 for the first time in 1995 after DDLJ but he had to compete with Aamir, Salman and Sunny for the number 1 slot. He was number 1 prominently till 2007 after which Aamir and then Salman Khan.
After a lean phase Amitabh Bachchan made a comeback as character actor with Mohabbatein and hosted the hugely popular KBC. SRK after his lean phase made a comeback as lead actor with pathaan and Jawaan.
Amitabh has been a failure as a producer with his ABCL which bankrupted him. SRK has produced most of his films under his own banner and has been more successful than Amitabh. However SRK has been a failure as a host on TV. As an actor SRK may have worked great in TV but as a host his shows like Paanchvi Paas, Zor ka Jhatka, Sabse Shana Kaun had all failed. Even his season of KBC was also moderate only.
The two superstars cannot be compared because of different styles of acting (Amitabh being a much superior actor), personality (SRK is brilliant and witty orator, Amitabh is also well in articulation but SRK can win any debate), filmography (Amitabh has experimented a lot with his film in his second innings, SRK has by and large remained in his comfort zone and his experimental films haven't worked for him). The comparison between the two is like comparing apples and oranges. See the way both actors have approached Don (both have done very well but they are not the same character really the way they have presented it).