The bag of feed has instructions either on the back of the bag or on the ingredient label. Every manufacturer has their own recipe with ages their feeds are intended for. It would be much better to follow those instructions than get advice from someone that doesn't know what is in the feed.
There is little difference in either feed. Vitamin and mineral content will be nearly the same. Primary differences are protein % and the size of the crumbles. To a lesser extent, fiber and fat content.
Some manufacturers, like Purina make a start & grow. That is intended to be fed from hatch to POL.
Don't feed layer to 8 week old birds. Calcium content is too high for their kidneys.