Depends on a lot. I'd consider taking both to level 31 for similar resources to maxing out one. At level 31, you reach the fast attack breakpoint for mud (assuming your 98% is 14stamina). You gain little after that breakpoint. Obviously, you'd want both to have solar beam.
After Kyrogre, you would then charge TM one of them to earthquake and can max them out as you get the resources to spare. An Eq Groudon and a Beam Groudon are functionally two different Pokémon and you may want both. It is unlikely any ground type every tops Eq Groudon. Solar Beam Groudon is arguably the top grass attacker we have. Having one of each is worthwhile.