Just pulled my second Scarlet, I have one at 3.60 plus base costume. I also have Kelie at 2.1. Should I level my 2nd Scarlet and use her and her costume on my red team or finish Kelie? I also have Khagan but no mats for him past 2.60…what should I do? FYI: none will be on my defense team, strictly for offensive: raids, war and events. See below my hero rosters. All advice welcome. 
SKip Khagan for now; a 3/60 4* hero is better than a 2/60 5*.
Personally, I would go for second Scarlett because you do have her costume. Although the costume’s attack buff will overwrite with Boldtusk and Mack, at least the poison damage is unique. Most importantly, you can benefit from the costume buff (whereas Kelile doesn’t have the costume).
Just my thoughts anyway Not sure if you use mono (all-Red) team, or you mix them around.
I typically choose diversity over duplicates. Kelile is also one of the hardest hitting 4* snipers.
There isn’t really a wrong choice as much as a preference. As @sleepyhead pointed out though Scarlett will have better stats than Kelile due to costume bonus. Maybe this will help to show what Kelile can do though as a sniper:
I am a big fan of hers.
