First and foremost, you should always pick a feed and stick with it. The constant change is hard on their guts. If your rabbits are doing well on something don't switch because something is cheaper. If you are going to switch, switch because it is a better feed or because you can't get the other one anymore.
Fiber is an extremely important factor. Anything below 18 is really too low. Rabbits can do fine on lower but for the larger breeds many think they do better on the higher. Fat is also important. A minimum of 2.5- 3 is better.
Protein is another extremely important piece of the nutrition puzzle. A giant breed such as the French Lop should be on an 18% protein blend. In some areas this can be hard to find in a quality feed but for their development and overall health of your giant breed rabbit.
I personally struggled with finding a feed that worked best for my rabbits. It was a nightmare for a while. My two French lops started losing weight and there was nothing I could do to fix it. They were eating fine, getting treats and exercise and still, dropping weight and deteriorating. I found out that they needed a feed with 18% protein in it and most feeds only contain 15-16%. I was feeding a feed with 15% and it just wasn't enough even with treats and fresh veggies daily. Once I started learning more about it from experienced breeders they turned a major corner and started gaining their weight back and becoming my healthy and happy rabbits they were before. Most people are unaware that not all feed is equal for each breed. Their size, activity level and portions all determine which feed to choose for your rabbit. The issue is how do you know what your breed needs? If you are buying from a breeder, they should inform you what to look for in a feed for that specific breed as well as send you a small bag of feed to help transition your new rabbit over to what ever feed you decide to put him on.