I think one other consideration is how big a feeder you need in relation to how many bird you have. Yes a huge feeder would save some storage space, I suppose. But if you leave feed out in damp weather, even under cover, for the longer time it would take a small flock to eat it, you run the risk of it going stale or mouldy from the high humidity. I use a plastic Solway feeder which holds 2.5 kilos and my 4 hens need two fills per week. The feed itself is kept safe in a dry, rat proof metal dustbin, which I feel would be a cheaper and better investment than a treadle feeder. Definitely agree with what has been said about wild bird contamination - my girls are in a roofed run so don't come into contact with birds, but my friend nearby says starlings are eating all his chicken food from the feeder inside the run when the door is left open for the hens to free range. So rats could also be the culprit. The only fully effective answer for a small back garden flock, it seems to me, is to have a run which is big enough for the hens, and to which rats, mice and birds have no access. This will not only eliminate feed losses but also protect against parasites dropping off the wild birds, such as Northern Fowl Mite and redmite. I let mine out into the garden in the afternoon when they have all had a chance to lay, and shut the run door behind them, so when I call them an hour or so later they are keen to get back in and have a handful of corn and some more feed.
But whatever feeder you get, the most important thing is to put it in a dry place. Even those which purport to keep out rain are not fully effective in drizzly or windy conditions where the rain is driving or blowing in sideways, and a very little amount of damp will spoil feed if its left hanging about in large uneaten amounts.
