I'd think of BA Premium Economy as being like Economy might have been sometime in the past when it was more expensive and more comfortable. The degree to which WT+ is worth extra value over WT is in my opinion largely dependent on your size. Someone who's a petite 1.45m would obviously benefit less than a 1.85m slightly overweight businessman. It's maybe worthwhile saying that World Traveller on BA's 787 is particularly squished, so WT+ is an especially good upgrade here.

I've never flown SAS Plus (to be honest I hadn't been aware it existed), but the seat dimensions and legroom match the BA ones pretty closely...