WV, good choice on the Varun from those three. It's a great down-river boat that's still very playful.
As for the other two boats, true, both will do you fine. I haven't paddled either, but I've been on the water with many and have talked to their owners about them. From what I know, the Karnali is most often used as a dedicated creeker. Iv'e heard that the Mamba does really well in big water, but (at least the largest size) is difficult to roll.
I hope that little snippet helps! Have fun.