The difference between slowbro and slowking in the main series is just swapping defense and special defense. It's unlikely they will have different stats after any rebalance. Also their typing is exactly the same
So you only care about their moves. For gym defense, Slowbro is better right now because of Ice beam. For offence they are identical (as a Psy atacker). But they are a bad choice for offence anyway.
You could wait a couple of weeks and see what happens after the rebalance, there might be something changing in defender mechanics. Movesets could change as well
