Spike & Drusilla were the ultimate vamp pair:
sexy, devoted to each other, deliciously evil together, unapologetic, they complimented each other

Angel & Darla never really seemed to care about anyone else until it was way too late. Both characters were so convincingly selfish & self-centered, I never bought their supposed love affair for one minute. The two didn't even look good together, while Spike & Drusilla's blonde & brunette reminded me of Yin/Yang. Also, most of Angel&Darla's romance is shown in flashback while Spike&Dru are often shown being a couple in the present tense. So their bond appears stronger.

I know both guys wind up falling for the Slayer but Spike's one remaining slice of humanity even in Vampire form was his love for Drusilla. Angelus was a convincing sadist but not a lover.

The Spike&Dru episodes were always exiting and fun but some of the the Angel&Darla shows in Angel often slowed down things and got too soap opera like.