The two biggest reasons for me to go with the Manbug instead of the Dragonfly are 1) size, and 2) cost. I love the Dragonfly design; I don't think that anyone could argue that the MB fits the hand better than the Dfly. But for me the Manbug is too small to carry exclusively, and if I'm going to carry two knives, I want the secondary knife to be as small and light as possible.
Cost is the other thing. Sure the Dfly might only be 20 or 30 dollars more but add that up over the cost of 3 or 4 knives and suddenly it's a $100 difference. You can buy two Manbugs for that.
Of course they're two different knives and you can't really do direct comparisons. In my opinion, the Dragonfly is no less a genius design, and I'm still on the fence about buying that beautiful Pakkawood HAP40 exclusive. Probably the best looking Spyderco I've ever seen.
How about a Pakkawood HAP40 Manbug?