Onyx,
Sumburgh or Scatsta?
Scatsta isn't used for scheduled services anyway, bar the scheduled crew change oil charters. Personally I do not know why they can't use SCS in some more enhanced way for schedules, as (from memory) it's closer to Lerwick than the 40-odd mile trek to & from LSI. Although I don't know how much it would take to carry out any upgrade/further upgrade at SCS, & I'm unsure of the topography around the place. Shetland doesn't have any high 'hills' as such, but the land around might still be challenging...not to mention the weather!
I doubt very much Sumburgh, as KLM would just use ABZ & passengers for LSI change at ABZ.