After waiting so long, can you wait another month? Groudon is back next week, Mewtwo next month.
The IV’s of what you have are great IMO, those fractions of a percent with the 12/13 will truly make very little difference in any given raid.
Personally, I’d wait until you’ve raided out Groudon and Mewtwo before deciding. You could lucky trade your Groudon tomorrow and then catch a shundo next week. This includes, of course, on your alt so if you catch a shiny on your alt, you could then lucky trade the shiny.
Neither Groudon or Mewtwo are the most useful of types, but they are the best of their type currently so it depends what you think you have better coverage in. We have one msystry raid boss coming, but it’s only a week or so and even if it’s something one or the other is SE against, it’s not like you’re missing it for a full month’s run.
XL candy, especially XL legendary candy, is yet to really sort itself out into any truly viable source and so if you’re looking that far into the future and you plan to XL Mewtwo, maybe go for the Groudon now?
I suppose a final consideration is this even though you say ETM’s aren’t a problem: There’s no mention of Groudon having Precipice Blades yet and so, Mewtwo next month having Psystrike, using an ETM on your traded Mewtwo will leave you with something, at least move-wise, you’ll be able to catch freely soon.
If it was me, I’d wait but if you’re determined to make the trade now and wanted an answer, I’d personally lean towards Groudon. You’ve got 30-40 more chances to go lucky with your alt again before the M2 run is over.