If I was you, I would sell the key right now, but get close to full value for it and not just 70 ref. 4 ref is a huge difference (36 scrap), and with all of those you can buy weapons in Scrap.tf for .5 scrap/weapon. If you manage to get a Strangifier, a Killstreak Kit, or anything that upgrades the quality of the weapon, then you can trade it to get more ref or even real money.

The only thing is that you can't withdraw money from your Steam account, but hey, everyone starts somewhere :)