Yes Globespan owns the 600's and took an opportunity to sell them back to SAS, why? well the current issues with SAS fleet (dash 8 landing gear stess problems) plus if you think about it. 737 is a better aircraft to operate than the dash 8 anyway (feel free to correct me on that if I an wrong).
In some ways yes, but when what if the demand on a route is for say a select business type that needs quick frequent flights (UK domestic etc.) with a demand for 50-80 PAX per flight - how would a 736 be better than a Dash Q400? Or what if the runway was only 4000ft long? The list goes on - it is pretty impossible to compare the two a/c.