MS/Eq vs MS/SB. Both currently and long term.
Currently: SoB - but only if you don't have better grass attackers
Long Term: EQ (or hopefully Precipipe Blades) - highest base stats among all generations for ground Pokemon
Groudon is best in one thing and that's ground, I would only (temporarly) use him with SoB if I would lack grass attackers - which, with today's update, gained a lot of use due to 3 Lv3 and 2 Lv4 Raid Bosses being doubly weak to it
Groudon faces some competition using Solarbeam and faces no competition when using Earthquake. Solarbeam has more neutral damage, but there are far better generalists out there. Water types that Solarbeam takes out are Super Effective against Groudon anyways, though it makes a great counter to Rock/Ground types for resisting Rock moves.
Currently: Makes a great counter to Onix, though Kyogre will give it a run for its money. EQ Groudon is better-suited for neutral game against non-Solarbeam Latios if you're running short on counters. Solarbeam may edge out EQ against TTar, but EQ destroys Solarbeam against Aggron so the jury is out. Groudon faces competition from Solarbeam Exeggcutor, Leaf Blade Sceptile, and the new Frenzy Plant Venusaur. It is somewhat stronger overall but its niche is kind of filled, and its main niche (Solarbeam effective against mons that are also weak to Mud Shot) is essentially against Rock/Ground types and Water/Electric types only. In the former case, Kyogre does this job a lot better, and why are we even talking about the latter case haha.
Long-Term: Grass types may see competition from mons like Tangrowth. Groudon may be rivaled by Rhyperior in bulk but in damage and overall Groudon will probably be better, unless Rhyperior gets access to far-superior Ground moves than Groudon does, for whatever reason.
In short, I think Groudon is best as a Ground specialist because Ground takes out so many targets. With Solarbeam, you lose Groudon's capability to take on Fire and Steel types in exchange for better coverage against Rock (commonly also Ground-subtyping) types and lackluster substitution against Water types, as well as increasing Groudon's generalist capability (somewhat). I would stick with Earthquake.
