Marvel and DC came of age at different times. DC came around in the fifties where everyone was Mr. and Mrs. perfect living in suburbia surrounded by white picket fences. Its natural that such an optimistic time wouldn't relate to a modern standpoint. Marvel is simply younger and more cynical, which is why most people in this day and age would pobably fiind it to be more believable. But then again, the Big planet destroying ubervillan of Marvel is a guy wearing a Pink helmet and a dress...apples and oranges.
I'd say Marvel's more realistic, only because of the personalities and thinking of the general public. From the beginning the majority of the world has never fully trusted and has feared any superpowered beings, and had taken steps to deal with them, which is very a very realistic view of how it would be. Although, DC is doing a very good job of doing that now.
Possibly , but were talking on a possible set of realism , who knows in 50 years splicing may be possible. What were pointing out though is things which are improbable and impossible because they break the laws of physic's (Eg Cyclops's powers will never happen , no mutation would make that possible , wheras angels could happen if nurtured correctly)
That really happened with mutants though. I always found it unrealistic how the general public hates and fears the X-Men but absolutely adore the Fantastic Four.
Outside Spider-Man, Wolverine and a few others there would be no Marvel either.