Chelsea weren't in the CL or EL when Conte made them champions, it's incomparable.
Sterling didn't play in a long international summer tournament like Mane. AFCON started Juni and finished on 19.07 with Mane playing in the final after playing in June with Liverpool in the CL final. Sterling's season finished earlier and he had 2-3 games with England. Despite having much more rest than Mane in the summer, Sterling has experienced a dip in form since November. Mane has been as good in April, May, June, July, Ausgust up to January. Indestructible and absolutely consistent without having much rest at all.
Edit: Last season, in the PL and the CL, Mane played about 600 minutes more than Sterling which is more than 6 full games. Sterling had more games in the cups but there the physical and mental effort isn't comparable. After that taxing season Sterling had much more rest and still hasn't looked as consistent as Mane.