I personally prefer Spanair over SK, just because the FA are usually cute, younger and spanish, instead of the usual old danish/swedish ladies (just personal preference.Some care , others not). The food is slightly more consistant, but otherwise no big differences. Same crappy MD8Xs planes. No idea of lounges of spanair in Spain. Domestic inflight service within Spain is reduced to a chocolate, but apparently they have a kind of business class, even for short hop as MAD-ALC.

The big drawback taking spanair instead of SK is that your miles are not qualifying, even if booked through and with SK codes.
SAS could really change that rule (as it is done in oneworld) to at least earn miles in your own program (i understand it's impossible for others * programmes) if tickets is sold with SK code.