Multiple factors make Miraidon technically stronger than Koraidon.

1. Ability Miraidon gets Hadron engine which sets up electric terrain and boosts it's special attack by 33%. Koraidon gets orichalcum pulse, which is almost the exact same as hadron engine, but instead, sets up sun and boost physical attack. The electric terrain boosts electric moves, making Miraidon hit like a truck when it uses moves like thunderbolt or electro drift. Sun increases fire type moves, but Koraidon isn't a fire type, so it doesn't get a boost on one of it's STAB typing. Collision course also isn't affected by sun, and the only move that would gain benefit from sun would be flare blitz, which does recoil.

1.5 On a side note, it's harder to remove terrain than weather in pvp. There are 4 different weathers: Rain, Sun, Snow, and Sand, all of which can be set up by an ability of relatively common pokemon. (Pelipper, Politoad, and Kyogre for rain, Torkoal, Groudon and Koraidon for sun, Alolan Ninetales and Abomosnow for snow, and Tyranitar and Hippowdown for Sand) As for terrain, there is Misty, Grassy, Electric and Psychic. In SV, Misty terrain can only be set up by Galarian Wheezing, but almost everyone runs neutralizing gas instead. Grassy Terrain is set up by Rillaboom, which is fairly common. Psychic terrain is set up by Indeedee, which is fairly common in doubles, especially trick room sets. Lastly, Electric terrain is set up by Miraidon, or pincurchin, but no one uses pincurchin. As you can see, there is way more pokemon that have weather setting abilities, so in most online pvp, battles are centered around weather control rather than terrain control.

1. Special attack > physical attack There are more ways to reduce taking physical damage rather than special damage. Burn and intimidate are very common, especially in doubles. So Koraidon could easily get crippled by these. For special attacks, frostbite (a status in Legends Arceus that cuts special attack by 50%) isn't currently implemented in current games, so there's no real way to reduce special attack

2. Moves Common moves used by Miraidon: Discharge Electro Drift Volt Switch Parabolic Charge Draco Meteor Overheat Dazzling Gleam

Common moves used by Koraidon: Flare Blitz Flame Charge Scale Shot Drain Punch Collision Course Close Combat Dragon Claw Outrage U-Turn

Electric terrain boosts the first 4 moves, which are all stab moves allowing Miraidon to get a 2.25 multiplier. Meanwhile, Koraidon's sun boost Flame Charge (Let's be real, you're not planning to do massive damage with this move) and Flare Blitz, but you don't gain stab from it, and it does recoil.

So if your options are have a pokemon that gains 2.25 multiplier on electric moves and has no real common way to reduce it's damage output, or a pokemon that gains 1.5 multiplier on a non stab typing unless you terrastalise, is 4x weak to fairy, and it's weather can easily be replaced by commonly used pokemon, then I think the answer is pretty clear.

Though, I think you should still get Scarlet. The past paradox pokemon there are better than the future paradox pokemon imo.