Like I said, no data should be necessary. In almost every familial situation the one scenario is going to be more difficult for children to handle than the other. But, yes, both scenarios would be difficult to deal with.
Allen's behaviour in the early 90s with Soon-Yi was extremely selfish, inappropriate, and creepy (and Soon-Yi shares some of the blame in that). Some of his children found it to be a huge mind fuck, something they still struggle with (although one came to terms with it and has good relations with him). They've said as much. Putting aside hypothetical scenarios, those are the specifics here.
Here's the thing about the Allen-Previn affair/marriage, though; there have been others like it-
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- Charlie Chaplin & Oona O' Neil-met when she was 17, married a year later (he was in his mid-40's)
- B. D. Hyman-daughter of Bette Davis, met and fell in love with David Hyman when she was 16 and he 29
The match-ups above have gone on for a long time (in Chaplin's case, up to his death) and the Allen Previn marriage is still thriving, so make of that what you will (mind you, I'm not advocating this kind of thing for everybody but sometime s, it works, and Previn was in her early 20's when she married Allen, so..)