There is a distinct different between the ACM panels you speak of. (Aluminum Composite Material) as they are known in the industry.
The grade of aluminum, the thickness of the aluminum, as well as the polyethelyne core they are laminated to. On some of the cheaper stuff, if you bend the corner it will actually break the aluminum and the core. Dibond will break if you bend it fast enough, but it mostly just bends before a failure.
Dibond is also the flattest material on the market. The others, not so much. However, 90% of the general sign work that most shops undertake where they are offering an ACM as the sign substrate, any brand will do just fine. Just know their limitations and handle accordingly. I always bill as if I'm going to use Dibond (the brand name) then determine the application. Are they going to be CNC cut letters, or mounted on a wall, or hung where the panels flatness and rigidity come into play.
So unless you are fabricating multi-dimentional, CNC Cut objects/projects out of ACM, then you will be just fine working with the other brands. Look into how many things are actually fabricated out of Dibond, and you will be amazed!
Dibond has a longer warranty. Something to consider.