Atlantic is fine - but there are some potential procedural issues in store if you are working with the international division. This division is not in Belize, the bankers work in/speak Spanish and any transfer of USD to Bzd even within the Atlantic family can take quite a bit longer than from Belize Bank Int'l to a local Belize Bank account.
That being said, the local branch Belize Bank is presently going through a reshuffling of staff and procedures, creating a bit of slow-down.
Until this recent set of changes at BzBnk, we've had nothing but good service there.
Many years ago we also worked with Atlantic Bank but when they deducted a hotel's entire payroll out of our construction company funds we lost confidence.
I think the point here is that both banks are pretty good. Neither bank is perfect. so - I'd go with the one that has the service package that meets your needs.