I am writing a revised answer as I completely misread the scale of the work and that changes my recommendations somewhat. I will leave my original response below this one, however, as I do feel it captures our thoughts if ever going large, say something like 4'x5'.
In terms of the sizes you mention, you could get away without crossbracing (which is generally frowned upon for other reasons – but that's another topic) as both of these materials are unresponsive enough to environmental changes that the usual cause of warping – changes in humidity – is really a non-issue. However I think perimeter bracing would be a good idea to facilitate handling, framing, and hanging. And to provide a little more rigiridty.
Between the two options, I continue to like the composite panel. It has better rigidity and is lighter weight. Plus at this point it is such a common panel that confidence in its performance has been steadily improving. While acrylic sheeting is definitely durable, and has an excellent track record for stability, it remains just a very unusual choice and would see no advantage using it unless its transparency was important. That said, if using either of them we would suggest using an acrylic dispersion ground. For both some surface scuffing would be recommended for maximum adhesion.
In terms of attaching a perimeter bracing, there is a listing of structural adhesives approved for Dibond – and which are likely good for other brands as well – that can be found on page 36 of this Fabrication Manual:
Buy also consult with the manufacturer with of the composite panel you plan to use for their advice.
Acrylic panel would be more difficult and complicated to work with in this way So another reason to likely prefer the composite panel.
Hope that helps.
Sarah Sands, Senior Technical Specialist, Golden Artist Colors and Williamsburg Oils
My earlier response is below where I misunderstood the scale involved
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Hi - We do not have direct experience working with either of these materials at this scale, so what follows is just some general thoughts. In terms of thermal expansion, rigidity and weight, the composite panel really does seem to be the better choice. If you look through this brochure, which shares a lot of technical information and does some direct comparisons with other materials, you will get a good sense of its advantages (at least on paper):
And here is a sheet on Plexiglas with similar technical specs:
Other reasons I like composite panels are the fact that they are used a lot for large scale pieces in the commercial graphics industry, including billboards, display graphics, and exterior signage, so there is a wealth of information on how it performs at that scale and how best to mount them. While acrylic sheeting is certainly durable, to get the same stiffness you would need to go 2-3x the thickness and it comes with a much higher thermal expansion compared to composites. Ultimately, acrylic sheets are just a much more unusual choice at those sizes, with perhaps the sole advantage being perhaps its transparency, if that was important.
In terms of sources for advice, I would reach out to the manufacturer or major distributor/retailer for these panels as they would certainly have experience of how these perform at that scale. And if you were willing to purchase a panel at that scale, you might take a look at Simon Lui, who fabricates large scale panels for a lot of artists in NY:
He has a mechanical engineering background to boot, so comes to supports with a wealth of knowledge about materials and how they perform. And even just looking at his designs for integrating the panel with wood framing running along the sides can give ideas.
In terms of acrylic sheeting, it can be primed with acrylic dispersion grounds, as Brian mentioned, and unless you truly need the transparency we would recommend that. For more information, also avail yourself of the resources from someone like Plexiglas
For both acrylic sheeting and composite panel you are working with very engineered commercial materials and the companies that make these really know the most about them - physical tolerances, how best to mount, the ability to span a large area unsupported, etc. Hope that helps. I can also say that we have worked with a lot more artists doing large scale pieces on composite panels then on acrylic sheeting and that alone says something.