BotRK is an on hit item...first and formost.
If your champion is largely skill damage based the BT is going to work much better...
The item shines on champions that have a lot of AS built into them or who have on hits in their kit and first build AS.
For example. Xin Zhao has a lot of AS built into his kit. He also typically builds some AS. While his AD ratios are decent, they aren't incredible either. So BotRK is a very good item on him.
Another example is Kog'Maw. He has other on hits in his kit. He also has nice synergy with BothRK in that he shreds armor. He doesn't have great AD ratios though and he builds lots of AS with serker grieves and hurricane. So building RothRK is great on Kog.
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