Finish out your start and grow. You can feed either to all your birds, all your lives.
The real way to compare? Set the Guaranteed Nutritional Labels side by side. You will see why many of us used "Start & Grow" when we couldn't find "Flock Raiser" during the pandemic, or "Flock Raiser" when we couldn't find "Start & Grow".
Lets pick on Purina, shall we?
Start & Grow
Flock Raiser
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Flock Raiser is actually the superior feed, based on headline numbers. In my view, the only reasons to buy S&G are:
Flock Raiser is out of Stock
Flock Raiser is only available locally as pellet (if you have chicks)
You need Medicated (FR is available Medicated, but I've never seen it stocked)
You have a tiny flock, and 50# of FR will go bad before they can eat it, while 10# of S&G will not.