First of all buying of a 5th generation fighter will be a geopolitical pressure game for India for both the fighters... Secondly, in my personal opinion, if buying any becomes an essential then India should go for the F35 fighter but, it should be on the following terms like: I) Integration of AIM 260 , II) Integration of Indigenous weapons, III) No enforcement for de-activation of S400 systems, IV) Allowance for using it against any of our adversaries ( excluding USA ) in any condition, V) MRO facility in India, VI) Assistance in AMCA development, VII) No enforcement for buying other 4.5 gen fighters like F21/ F16/F15 etc.... In my opinion, Su 57 is not a complete 5th generation fighter as per as the definition.... But the negotiation tactics of India with USA will be tricky & tough by having Su57 as the closest competitor