The 1300 GSA is noticeably better quality and better screwed together than the 1200 for not a whole lot more than my 1200 cost me 8 years ago. I never had the 1250, but I imagine it was similar to the 1200 as far as dressing went. There's very little plastic on it, it's all steel and aluminium (tank).
My 1200 never gave me any issues in the 8 years I had it and it had plenty of electronics. Having said that, it never saw a pressure washer or salt and it lived in a dry underground garage all it's life, albeit on the coast. It's also true that mine was one of the last of the 1200s and very sorted.
In Spain these now come with a 5 year full BMW warranty because it's one of the few ways they can compete with the TEMU machines. I don't know if it's the same in the UK. If there's anything inherently wrong with the electronics or the hardware, then there will either be a recall (which makes a warranty academic), or it will likely show in that time. If not, well, yeah, it's a few grand for something big like the Adaptive Ride Height (ARH) or the suspension as a whole. But do you know what? It'll be worth fixing.
The simple fact is you cannot buy a new machine without a full suite of electronics these days. It wouldn't get past emissions controls for a start. You don't have to have the ARH either.
However, I think if I'm really honest, if I were to take any one of the two into a desert or across continents, or around the world, I'd take a an 1150.