Six kilos of cucumbers and one fridge shelf, salt ferment or vinegar pickle if I want them to last to Christmas

I do both every summer so I can compare directly. Refrigerator vinegar pickles stay properly crunchy for four or five months and taste of exactly one thing, which is vinegar. The ferments taste far better for the first six to eight weeks and then soften noticeably, and by February they are only good for chopping into salads. If December is your target, vinegar wins on texture and ferment wins on flavour.

For cucumbers I go three and a half to five percent salt by weight of everything in the jar, ferment five to ten days at eighteen to twenty degrees, then straight to the fridge. Two things decide crunch more than salt does: trim about a millimetre and a half off the blossom end, because the softening enzymes live there, and add something tannic like an oak, grape or blackcurrant leaf. Small firm cucumbers only, and get them in brine within a day of picking.

I fermented six kilos of fat supermarket cucumbers once because they were cheap. Every single one went hollow and slimy inside while still looking fine, and I found out afterwards that big waxed cucumbers are the wrong fruit for this entirely. Six kilos in the compost and a very quiet week.

Ferment the small ones whole, slice the big ones and put those in vinegar. Nothing keeps a fermented cucumber crunchy past midwinter, so stop trying.

Your real constraint is the shelf, not the method. Six kilos of cucumbers is three big jars plus brine, which is most of a fridge shelf on its own, so decide the volume first and then the method. I would ferment two kilos to eat before October and vinegar the rest sliced, because sliced packs a lot tighter.