I was in the same situation not so long ago. Plus is the same as economy with other airlines. Go is sort of an 'economy minus'. Only if your ticket reads booking class C or Z (full flexible plus class) you will get on a business class seat with a partner airline. All other 'plus' classes translate to economy.

I agree with fassy: claim your original flight plan for miles and points or whatever you like to call them.

The offer you got from SAS is the standard offer. Check if it holds according to the EU regulation. In my case it was sufficient. I'd also choose the voucher, as FlyingMoose suggests.