This thread isn't specific enough for me to answer, or maybe I'm not thinking right but... it really depends who you give that side B to. Someone like MK will probably benefit more from cape than DB since he already has enough moves to punish, but someone else like CF could probably make better use of the DB than the cape. Imagine Ike with pound(really hard lol). It'd easily be one of the best moves he could get.
But if the move is for its respected character, than well it's real tough. Some characters need their side B far more than others(ex. Wario, it saves me ALOT). Again, I'm probably either thinking too hard or not right, but this is how see it.