The standart Key is the Mann co key. If someone wants keys, they mean this one. Other keys are roughly the same value, slightly more or extremely more worth.
Scrap.tf also consideres only the Mann. Co Key a currency, and other keys just items. In auctions, the former is always worth 1 Key, while the latter may loose value as they have to go through the hassle of selling the item; how much value is lost there depends on the item factor.
Lastly, Mann Co. Keys are traded so often that most trading-sites have an special places to trade just them. Scrap.tf has an extra "Key" trading bot, while all other keys are found in "items".
I'd advise you to always try to sell your items for keys and metal and then trade with that, unless you think someone will pay more for your items.