Personally and I say this as someone who is, as @captpepperjack says, “swimming in stardust and larvitar candy” I’ve lucky traded a shiny larvitar for one Mega- Ttar and the other is a hundo that i caught somewhere special, but if it wasn’t for that, I’d be lucky trading another shiny larvitar for it. As Megas do really want to be well powered up, the lucky 96% for me is better than the non-lucky hundo, because of the saving. For me, the 10% candy saving is all but worthless, but I appreciate I’m luckier than some in my massive stockpile of resources.
For me, there’s two potential excellent shadows there and assuming you have no other teams at all, 3 each of Darkrai and Absol powered up even to L25 will do perfectly well as dark attackers. Without SD TTar falls right back in the rock charts so there’s probably a lot of better options, even if they’re only stop-gaps, Terrakion is back next month for example, assuming nothing else materialises in T1/T3 raids.
Mega Ttar won’t be out for a while I suspect, I think with it being part dark type, Halloween was the chance, the next will be an ice type at Christmas I suspect (Frosslass?) so there’s loads of time.
TL;DR what @stativision said, hold on until you need one or the other. But make sure you TM them both if/when another rocket event ever happens. At least you can evolve Pupitar for the SD Tyranitar if it comes before the next Frustration-TM event.