I love my Darwin. It's one of my favorites. The only thing I would change is its aesthetics, but I had it refinished and painted so it's not too hard on the eyes now.
I love the 5v GLV-2, also. I have two of them and they rock for fixed voltage. Say what you will, but second to the GLV-2, I love my V2 Maxi Roughstack. Runs 3.7, 6 and 7.4 volts, durable as heck and lasts for a long time. I take it rock climbing and camping. Definitely my two favorites for fixed voltage.
As for the price, I think it's decently comparable. A satin ProVari starter kit with batts, a charger and extended warranty will run you $200, $220 with the extended cap, or $160 for the bare bones unit. Last time I checked Nhaler's going rate for a Darwin is $245. I like them both, and it'd be hard for me to choose one over the other. The only thing, in my mind, that gives the ProVari a slight edge is that you can bring extra batts with you, while the Darwin has to be charged as a unit. I do love the variable wattage/power function, though, and I'm extremely excited for the Kick.
All in all, OP, if you go for any of them you won't be disappointed. What turned me off from the Lavatube were the reported durability problems and issues with the automatic adjustment. If memory serves, GrimmGreen did a review on some of the Lavatubes' adjustment issues on YouTube. Very informative.