Here's a few thoughts from me ....
In the January 1969 version Paul is singing and playing bass at the same time, so what he is playing on the bass will be fitting in around what he is singing.
In the version recorded in April 1969, however, John is playing the bass without worrying about having to sing a lead vocal. So the two styles will naturally be different.
Also, John's bass line seems a bit more "obvious" to me - it's probably something along the lines of what I would play if I was asked to play Oh Darling and I hadn't heard it before (Like Lennon, I'm a guitar player not a bassist).
A good bass player like McCartney, on the other hand, will do stuff that is not so obvious but which complements what is going on around it.
And finally, I've been told by bass players in the past that when guitarists try to play the bass they always do too much - a good bass player is happy to leave a silence now and then, if it fits the song, but a guitar player will be trying to squeeze notes in everywhere!
If you listen to Let Me Roll It, for example, there are parts where Paul plays a similar line to Oh Darling, but there are other parts of the song where he is playing virtually nothing and it suits the song perfectly.