I haven't paddled a Mamba, but I paddle a Karnali L and it is the best creek boat I have ever owned. I have paddled it on everything from small steep creeks to huge volume rivers and it handles it all with ease.
Positives: (1) It is fast and takes only a couple strokes to get up to speed, (2) Unbelieveable secondary stability- I literally have to throw my weight to get it upside down (3) Holds a line well and plows down small waves and holes- you do not even feel Class III sized features (4) rolls easily (5) great for overnights- you forget that gear is back there.
Negatives (1) It doesn't have quite as much rocker as other creek boats, so not as much of an autoboofer- but I am guessing this is not a big deal on the New River (2) the outfitting feels less solid that some, it wiggles around a bit.
If you are unsure about sizing, I would go for the large. I am 6'1" 160 and it fits me fine.