If you are ageing a wine for 18 months or less, it is probably better
to bottle under screwcap as it protects the wine from oxidizing as
quickly, plus there is no risk of cork taint.

For long-term ageing, the jury is still out on scewcaps (they simply
haven't been used on age-worthy wines for long enough). If you have a
big red you intend to age for years, you might want to bottle a few
under screwcap and the rest under natural cork and test them every year
side-by-side to see how they develop differently.

Most of the studies I've read indicate that the screwcaps preserve the
wine much better, but can give off flavours after a period of time.
And of course, if you are ageing a wine into maturity/austerity,
preserving it isn't exactly the goal.