I say both size & strength! who wants to have big, weak guns?
hammer curls are good. do some barbell/EZ barbell curls also, and sometiems swap those with normal dumbbell curls (rotating the palms from neutral to supinated by hte peak of the rep)
remember your back exercises (especially rows and pullups) hit the biceps too.
I'd stay within a 5-10 rep range...lower reps for strength, slightly higher reps to stimulate growth. maybe alternate weekly or every other week on the rep ranges.
most important: don't cheat! the shoulder shoudlnt' move...the elbow shouldn't hardly move...doing this isolates the bicep.