What's the best format to export out of Metashape Standard, for viewing a vertex coloured shaded model, or the textured model in Viewer?
The real step-change for the viewer is the ability to measure, interrogate and analyse the model and for this a scaled and (optionally) geo located model is required.
Apart from "It looks like this" - which in itself can be of some value - the standalone viewer can't really do much more than online viewing tools when it comes to handling Metashape Standard output I'm afraid.
Opitmising images, scaling, geo referencing and the accompanying outputs are what the Pro version delivers and what the Viewer is now designed to handle and work with.
As for what format...OBJ, FBX...whatever really. Some are physically smaller than others, but not tested every variation.