Honestly my main recommendation would just be to make your first SBR a standard AR lower regardless of the caliber you choose to get it in. This will just give you the most options down the road for your tax stamp.
That being said, of the options on your list I would personally go with a 10.5" 5.56 with an adjustable gas block or an 11.5" one if you don't want an adjustable gas block.
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