A2S is worth checking for special consolidator fares, particularly international flights. If significant, go with A2S.
Other than that, you are better off booking directly to maintain control of your booking. It is far easier to make changes directly on the airline’s website than to have to call and do this through a third party. Also, inside final payment, A2S will charge a $$$ penalty for cancellations/changes even if the airline would not charge a penalty.
”Assured arrival” is misleading marketing. Like any travel agency, they will work with you to rebook missed/cancelled flights. That’s all. If you’ve been led to believe otherwise, consult A2S’s terms and conditions.