I asked that same question nearly a year ago. It started quite a debate, lol. Here is a link to the
thread I started so you can sift
through the results I had.
http://www.e-cigarette-forum.com/forum/requests-opinions-reviews/177849-vary-not-vary.html
I'm sure others will pipe in here and give you their opinions. Both the Provari and Darwin have their fans who will argue that their choice is absolutely the best. Personally, I went with the Provari and I'm extremely happy with it. I've never had any problems with it, which for the price is an important consideration. I have also discovered
tanks which I now use exclusively with my pv. Although many Darwin users will tell you that you can use a tank on a Darwin, it just seems to me that they make more sense on a Provari.
Some points to consider....I'm told that the Darwin battery lasts a long, long time. My Provari (with extended cap using an 18650) will only get me through about 12 hours. I've heard the Darwin lasts longer, but can't verify that. The downfall is that the Darwin has to be plugged in to recharge. You can't just swap out the batteries, which imo was a negative. Some folks don't see that as a problem though.
I will say that vv is definitely the way to go. There is a world of difference. Whatever choice you make, I am sure you will be happy with the results.