Get the 50-pounder...and get several if you can get find them on sale.
I keep one for each of my base malts along with a big 45 gallon plastic storage tub/tote with a lid that I can stack full of unopened bags if I stock up on multiple bags of malts like Pilsner and 2-row that I use a lot of. I can hold about a 6-month supply of base malts.
I just use generic 5 gallon buckets with standard issue lids. Small quantities are in their original packages which I tape shut after opening. Base malts go straight into the buckets. Everything is kept inside, and since we live in the high desert, humidity isn't a concern.
I use the 50lb vittles vault for base malt. Five gallon buckets, with lids, for smaller quantities and DME. Everything is stored in the basement. No issues so far. I really like the vittles vault and will probably upgrade at some point.