I'd say if you have good (i'd say even a 10/10/10 would be worth it, preformance difference of a 10/10/10 from level 25 to 30 and a 15/15/15 level 20 to level 30 arent that huge, if it was maxed out, i'd say always go higher IV's, but for a boost that small comparatively, saved dust/candy is more worth it) Iv boosted ones, leveling up one of each wont be a big loss if its to level 30, as they currently are level 25.
Kyogre seems the more relevant of the two to have, given its effectiveness against Rhydon raid, and if we get raid days of the beasts, will be great against Entei. I'd recommend powering kyogre up as its great against groudon right now unless you already have high level HC feraligatr
Groudon on the other hand, as I have recently discovered isnt as good legendary, and even tier 1-4 wise (unless facing electric type tier 3s), against dialga, it lags hard compared to machamp unless weatherboost. One level 30 will be more than sufficient I would think.
Their exclusive moves likely wont be out for a while, given niantics shall we say slow releasing of moves and event pokemon. For reference, we dont have Psystrike mewtwo, a gen 1 pokemon, and the weather trio are gen 3, we likely wont get them until after mewtwo, which could be a very long time.