I don't think it matters, IMO. Purina says you can keep them on Starter until they start laying. However, those of us who have mixed poultry flocks often use Flockraiser for everyone and it works equally as well if you supplement your egglaying hens with oyster shell.
I would opt for the higher protein level. Growing birds need high protein to grow all their tissues (muscle and feather to name a couple). As they get older and growth slows, then less protein is desired. Different purposes for chicks require different approaches to feeding them (i.e. egg layers vs. broilers, slow developing species vs. faster developers) but generally start/grow the chicks w/ more rather than less. Good Luck!
I feed mine Flock Raiser, 20%, and add oyster shells and grit, and I have six very healthy birds who are now 9 months old. I like it for its quality, protein content, they eat less because of its quality and I like the idea that the girls can regulate their own calcium need as nature dictates.
I would go with Flock Raiser but do not put calcium additive like oyster shells or egg shell out until they lay their first egg or they are around 20 weeks old. Their kidneys may not be able to handle the calcium. You can feed Flock Raiser for the rest of their lives. They will regulate how much they eat based upon their needs.
If it were me, I'd go with the Flock Raiser. If there were to be any signs of coccidisiosis then I'd switch them over to the starter (they have medicated and unmedicated sunfresh variety's I believe). At 8 weeks though if they have been allowed outside to play in the 'dirt', they should have built up their resistance to cocci by this point.
I read somewhere that the Purina Start and Grow is good for that, starting and growing of chicks until egg laying age. Now I also read that the Flock Raiser is good for heavier breeds, after starting them on the Stareter feed...after they get to 8 weeks...so 8 weeks -20 or so I suppose. I suggest going to the Purina Website itself to double check that! ; )
15-16% grower/finisher feed is appropriate. However, I prefer to give a higher protein feed like the 20%, and supplement with greens like grass clippings, clover, chickweed, lettuces, cabbage, etc. That way the youngsters earn to eat greens, but without getting their average protein reduced to lower than recommended levels.
I prefer for them to learn to eat greens early. It makes them better foragers. At laying age the better foragers and greens-eaters will give you darker, more nutritious egg yolks.