The way I would weigh the question is like this: which do you feel to be more beneficial to you? A piece of armor for one specific class or a weapon that can be used on more than one? While I think Obsidian Mind is amazing, I sometimes find myself using my MIDA during runs in the abyss on Crota's End, and that speed boost is no joke. Ultimately, if you have multiple classes, I might think about the Multi-Tool. If you only have a warlock, or it's your preferred class, then I'd probably lean towards the Obsidian Mind.