For a family with children, particularly school-age ones, a week tends to work harder for the effort involved. Packing for several people, planning meals, keeping children entertained: all of that overhead is roughly the same whether the stay is three nights or seven, so a longer stay spreads that effort across more days and generally works out better value per night too. A week also allows for the inevitable one wet day without losing the whole trip, and gives time to properly see more than a single nearby town.
Neither is a lesser option than the other, and mixing them across the year often makes more sense than picking one. A couple might take a short weekend break in spring or autumn and a longer week together in summer; a family might use a half-term weekend for a quick recharge and save the full week for the big summer holiday once school allows it.
August's bank holiday sits at the tail end of the school summer holidays, so it tends to be the busiest of the three by a clear margin, effectively the last big weekend of summer before term restarts. The May bank holidays fall outside the school holidays for most, so they draw a different mix of visitors, more retired couples and those without school-age children, and tend to be marginally calmer as a result, even though they're still busier than a standard weekend.
Evenings are where a winter break earns its keep. With little daylight left by mid-afternoon, the pattern shifts naturally indoors: a warm static caravan holiday lets, a proper meal, a board game or a film, curtains drawn against the wind outside. For a couple or a family who actually want to slow down rather than fill every hour with an activity, that enforced indoor time is a feature rather than something to work around.
The practical advice is the same whichever May date is the target: check the actual calendar for the year in question, since bank holiday dates move, confirm it against your children's specific half-term dates if that's part of the plan, and book earlier than feels necessary. May fills up quietly and then, in the specific weeks that carry a bank holiday, fills up fast.
The general advice holds across all three: if the specific bank holiday date matters, book as early as the diary allows; if flexibility exists, the days immediately either side usually offer a similar break with an easier drive and a calmer park.