Depends on what you really like, of course. Those are both good possibilities... Lots of people get really excited about the 6 line, but I could sort of take or leave ours.

You could also consider:
pajama cardinal
bangai cardinal
firefish
chalk basslet

I tried to consider the swimming areas of the fish you already have (watcman will stay on the bottom and clowns will stay closer to the top, so the ones I suggested are likely to swim in the middle or all over).