We've done SK ARN-ORD and BA ORD-LHR in economy and, if only comparing at a direct nonstop to my final destination, would prefer SK service. It doesn't feel as cheap and surly as BA, and I prefer "cold" Northern European workhorse crews to whatever stupidity/shabiness/inconsistency is offered from BA Mixed Fleet (which ORD is). I hope to never ever see a cabin crew tuck in the back of their too small shirt and then serve drinks with the same hand, without washing, again.
We have done SK Plus ARN-LAX and really enjoyed it - you could always try for the upgrade. I've gotten too old to sit in Economy that long anymore. But for a run to the East Coast, and maybe even to Chicago, I would happily take SAS in Economy if that was the only option.