They're really two completely different animals and both are awesome mods, so ultimately it will all come down to your preference
I've owned both and still own my Darwin. Now that the V2 Provari has a higher amp limit (3.5) it is definitely more appealing to me as that was one of my negatives about it. The Darwin I believe limits at 3 amps, but since you can't take the wattage above 12.7 you can still hit that max setting even with a 1.5 ohm atty. The battery hasn't been an issue for me in almost a year of using it, it lasts me all day and sometimes longer depending on what gear I'm using on it and how heavy I'm vaping (I can get two full days with CE2s since I use a fairly low watt setting on those, but only a day with anything LR like dual coils)
The menu system on the Provari can look intimidating, but it's really easy to get used to. Sure, not as easy as just turning a wheel and adjusting on the fly, but still not as much of a hassle as I was originally expecting.
Form factor and weight of the unit, the Darwin wins this one in my book, but YMMV since we all have different tastes
Hope some of my ramblings help you. Both are great mods and I'm sure you'll be satisfied with either one