Generally speaking, 15 - 18% protein range is appropriate for any age rabbit - baby on up to adult. 18% protein is a little better for a nursing doe, or a rabbit that needs to improve body condition. However, 15% protein works just fine in most cases. I've raised several does/litters and finished pens of meat rabbits to slaughter weight on nothing but 15% pellets and had no issues.
Some people are more concerned about the composition of the pellet than the protein percentage. Read the label. Some list dehydrated alfalfa meal as the first ingredient; some list forage products. The alfalfa meal pellet will be a little higher quality - hold together better, less fines/dust in the bag, etc.
I personally let my wallet influence my rabbit feed and haven't had any issues yet. I just blend the last bit of the old bag into the top of the new bag for the transition.
I buy my feed at Tractor Supply Co. Producers Pride 15% is usually the most economical. I do buy 16% Manna Pro when it is on sale and it is as cheap or cheaper than Producers Pride.
I am a backyard meat rabbit operation. People who show rabbits will have different opinions based on feeding their stock to be in prime show condition.