No brainer?

Dragon Tail has better neutral coverage despite lower overall power so you could consider FTMing it, as Fast TMs are usually pretty easy to come by even from only soloing low level raids.

However, ask yourself if the weather is frequently Clear where you live. If so, Mud Shot is the obvious pick. Also, for anything weak to Dragon Tail Groudon's Charged Moves don't really pair well.

If you go with Dragon Tail, you won't really get to benefit from type matchups at all (Dt is low energy, no STAB, and doesn't hit things super-effectively that Solarbeam or Earthquake would, despite being a really good Quick Move in a vacuum)

Depends on what you do with it. Against Kyogre it doesn't make much of a difference but you probably don't raid that too much. MS is better in a lot of Groudon's favorable matchups, soloing Omastar and Ninetales right now and later on against Regirock and Registeel. In gym battles MS is probably slightly better too.

I'd say yes, if you can spare the FTM. It's not a big upgrade like FTMing Machamp, Dragonite or Tyranitar.

I would.