Actually, the 21 did have a rear-ward facing AA laser, it was that yellow, antenna-like thing sticking out of its head in battroid mode, and behind the cockpit in fighter/GERWALK mode. Not to mention that its other two lasers can be fired backwards in fighter mode too. Another thing, the 21 didn't need FAST packs to carry its gun pods, they were fixed onto hard points on the leg covers.
But yeah I agree with you that the 21 is a great valk, but my vote has to go with the 22. All the pluses, well not ALL, but a lot of the pluses and great looks of the 21 with a few extras...