It depends on what is in the food, not the form it's in. Good ingredients with minimal binders, like flour, and ingredients that are suited to the fish in question. But 50% protein is too high for daily maintenance. If there are two foods, with identical ingredients, and one is flaked and one is pelleted, there is going to be very little difference to the fish, if any.
I like that you are concerned to feed the fish well. That's great, a lot of folks don't even think about the food. But many do not store fish food well, so you can do that. Keep only a small amount out to feed, store the rest air tight and in the fridge or freezer, so it remains fresh. Fats can go rancid in warm, humid conditions, and a lot of us keep our food right by the tank, where it is warm and humid. Rancid fats do damage, the oxidation makes them toxic.
You can always grind larger pellets down to a smaller size if you can't get them in 1mm size or smaller. A coffee grinder, the inexpensive ones that are like a little blender, with a small cup on top, work well for this. If you want to stick with one brand, fine. But not that 50 % protein.
And if you really want to feed the fish optimal foods, do think about culturing some live foods, or at least get some frozen live foods. Live food does wonders for all fish, brings them into breeding condition much better and faster, and is, lets face it, what they evolved to eat. No fish evolved to eat man made food.
But get something with a more reasonable protein content.
You are correct that there are arguments to be made on both sides, and much disagreement on fish feeding. But quality is always better than crap, and at least in the US the food has to have the ingredients on it, in the order of quantity in the food. In Canada,, we have no such laws for fish foods, which is criminal.
Most foods do have labels, because they are imported, we have only one or two manufacturers here. One makes an outstanding pellet food, Northfin. 1, 2 and 3mm sizes for veggie eaters, carnivores, omnivores. Excellent ingredients, no more binder than is necessary to keep it from falling apart after it's extruded, and they've had great results so far, but they are new, don't think they've been on the market a year yet.
Omega makes very good foods, pellet and flake.
Just fyi, they tried to substitute man made food in the shrimp farms, because baby brine shrimp, which is what the shrimp larvae needed to eat, got so expensive. They quickly learned that they can't sub' man made for all the diet. Mortality rates soared, which costs them money. They learned they can sub 70% man made but the rest must be live or the shrimp don't survive and those that do are not always good enough to sell for human consumption. So animals do better on food more like what they would eat if they were still living wild. I think anyone can see that if they think about it, the same is true of us. Something to think about.
I culture flightless fruit flies, micro and banana worms, infusoria, and soon will add ostracods, amphipods and copepods. I also culture pure green water, for my filter feeding shrimp and for clams, though the clams did eventually pass on. They needed colder water, I believe.
I feed live whenever I can, frozen, and flake, pellet and tabs, in turn, to provide variety. Fish are thriving, breeding, and look great. Just some food for thought, if you will forgive the pun.
You know, go back to earlier fish keeping and it was all about live foods, because man made did not yet exist. Those fish were amazing, bred and had great colours. Daphnia, even though they have little food value per daphnia, are eaten in huge numbers when available and can put colour and condition on fish like nothing else I have seen. I'm trying to culture them too.
