I think one of the reasons for choosing braided flex for irons was the added flexibility. PVC sheaths can be ridiculously stiff and that's annoying for something that's moved around a lot.
I definitely prefer solid for two reasons: most continental European (except Italian) manufacturers don't list their equipment for the use with stranded wire at all so you'd have to pigtail in each box (and use extra-deep boxes as officially switch boxes are rated for either one device OR connectors, e.g. Wago or choc blocks) and stranded wire has a greater outer diametre than solid because the strands are round and take up more space than one solid conductor. That's most noticable if you try splicing stranded wires using choc blocks.
In Austria and Germany everything for fixed installations up to 6 mm2 is solid and you can get 10 mm2 solid too. 10 mm2 and up are usually rigid stranded, each strand being roughly the thickness of a 1.5 mm2 solid conductor.
Stranded wire is occasionally used inside panels but rarely anywhere else.
There's no need to twist any wires these days, switches and sockets usually have push-in terminals (the last manufacturers to retain screw terminals were a few DIY-grade botchers where the plastic body of the sockets would give or even break before you could properly tighten the screws) and splices are exclusively made using Wago connectors - 273 series for solid up to 2.5 mm2, 223 for mixing stranded and solid. I haven't seen a professional use the once popular choc blocks for ages (except for connecting pendant fixtures) but I suppose a few conservative ones still might. 6 mm2 and up are usually spliced using fixed terminal blocks or connectors that fit on a DIN rail.
