I believe kegging or bottling is the safest for transporting the beer. Either are true sealed (airtight) environments which will prevent any oxygenation occurring from the beer sloshing around during transport.
Others may disagree, but I believe firmly that transporting already fermented beer in either primary or secondary has an improved risk of oxydation. Having an airlock and headspace in those vessels, sloshing could easily cause the co2 barrier to be breached, if air then gets in you are whipping oxygen into the beer.
So keg it, gas it, and move it confidently knowing that the beer will be safe from oxygenation. You don't hear about commercial breweries transporting their fermented beer in primaries or secondaries do you? No, the don't transport til the beer is in a serving container be it a keg or bottle.
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