Hello, it is so different and hard to compare. Present astbill do not have calling cards and main purpose there is p2phone and pc2pc. but there user have more choices to control his own account, let say (it is tricky), customer have something like freepbx.
A2Billing, instead, mainly for calling cards and user have less access. Would be nice in next version to let user (n user interface) to edit some settings on his sip and iax2 accounts (NAT, codecs, canreinvite, secret/md5 secret).
Anyway, i beleive, next or next after
A2Billing will be the best