I'm sure it differs by company, but I try to make sure I'm housing the documents all in one place - doesn't matter which you choose, but I prefer Marketo because of its organizing capabilities. Housing them in one place just ensures you don't miss any documents if they're ever updated
Of course, this could depend in your website's tracking capabilities and whatnot, so choose whichever system has the most benefits!
Agreed, it completely depends on your process. We keep most items on our website, but when it's a campaign specific asset then we use Design Studio. This has really helped as our company scales so that regional marketing managers can handle the asset localization themselves.