You've not indicated where you are. Shipping is one of the major costs, so finding a source in your area is important.
R panel is R panel as far as shape. Differences are in gauge, galvanizing, paint guarantee, etc. Red iron is highly similar from brand to brand. I would make sure they can supply whatever engineering stamp your AHJ might require.
There are several major players and most of them are members of MBMA:
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MBCI (MetalBuildingComponents) has several production areas:
Contact someone in your area and get the name of an erector. It is the erector that makes the big difference, no matter who makes the parts. Erectors don't typically care whose metal they run. If you are planning on erecting your own, this idea doesn't fly.