I would not invest resources in a 6 that's available as a 7*
Not true for all champs . My korg is r3 sig 150 and i am going to r4 him and max sig. A max aig r4 or r5 is miles better than an unduped 7* . Sig 20 7* is fine ..but not an unduped one.
Out of the two options, I'd go with 6* Korg since you'll want as high a sig as possible for him and you won't get that from the 7* anytime soon.
However, I would hold off from ranking up any of them until you have the means to commit those sig stones. I'm not saying that you have to have 180 sigs to pump into him immediately, just that he has to be a sig stone priority for you. If he isn't, then you most likely have other champions who'd be better to rank up.
I recently took my 6* Korg to r4 and sig 200, and have no regrets. I don't have a 7* but I would have ranked up my 6* even if I had a 7*. There's just a day-and-night difference between no/low sig Korg and a high sig one. He becomes so much more annoying and does so much more damage to attackers.
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However, I would hold off from ranking up any of them until you have the means to commit those sig stones. I'm not saying that you have to have 180 sigs to pump into him immediately, just that he has to be a sig stone priority for you. If he isn't, then you most likely have other champions who'd be better to rank up.
I recently took my 6* Korg to r4 and sig 200, and have no regrets. I don't have a 7* but I would have ranked up my 6* even if I had a 7*. There's just a day-and-night difference between no/low sig Korg and a high sig one. He becomes so much more annoying and does so much more damage to attackers.