SAS Go (light) and SAS Plus combined is pretty much every single booking class available, so it's not that easy to say from that. Depending on how far in advance you book, you'd likely end up in the 'cheaper' booking classes, for SAS Go giving you 500 points to EuroBonus, or 1.500 for SAS Plus. ARN-LHR is 911 miles, so finding a program that'd give you 100% for your go flights would add up to more miles, and likewise 200% would for SAS Plus. However 50%/100% would be less.
If you end up in the more expensive booking classes (super expensive especially in plus I believe), it gives 1000/2000, so you'd end up with less points crediting somewhere that gives 100/200%

A3 Miles & Bonus might give you more for the cheaper classes, but depending on if you can get the legs flown on A3 for their easier qualification it might not be easier to get status.
Have a look here: http://www.wheretocredit.com/sas-scandinavian-airlines and you can see for the booking classes you fly.

Also, keep in mind getting *G somewhere else than EuroBonus won't give you fasttrack.