I have both and use both on a daily basis, ProVari at home and at friend's houses, and Twists everywhere else. Here's a few (hopefully subjective) points to consider:
Price
Short term, the Twist is easily the most economical choice as a battery & charger costs roughly $30 at currently prices. Since you never want to go without a device and you need a backup, add an additional battery and the initial cost is about $55. These haven't been around long enough to determine the average life span, but typically the eGo battery life span is about 3 to 6 months with heavy to moderate use, from what I've been able to gather.
Long term investment is where the ProVari could be an advantage. A start up price for the ProVari is $160 for the base unit + $20 for 2 batteries + $25 for a good smart charger (XTAR WP2 II, for example), and the total is around $205. If you want the extension cap for the larger 18650 batteries, which I recommend for fewer recharges, then add another $20 to the start up cost. Battery replacements every 3 to 6 months (depending on use) will cost about $10 per battery or less if you find a vendor with good prices.
It would take approximately 2.5 years of replacing batteries every 3 months at current prices for the amortized cost of owning a ProVari to show benefit over the cheaper and disposable Twist batteries:
Twist - $55 start up + $40 every 3 months ($360) = $415
ProVari - $205 start up + $20 every 3 months ($180) = $395
Quality
Some will argue with my opinion on this point I'm sure, but that's all good with me. When it comes to quality, especially with electronics which are especially trendy, it's almost a wash. Yes, the ProVari is a very well made device, and that is evident to anyone who has owned one and has had any interaction with Provape regarding any issues. However, electronics are trendy and many would argue that longevity is not as important since they'll be upgrading within the course of a year or two to the latest device. This is true for smart phones, computers, tablet devices, and any new and awesome vape related gear that will be announced this year or the next.
With that said, my personal take on it is that quality does matter and each has quality points to be considered. Joyetech has been making good quality products for quite some time and has recaptured a consumer base with the Twist design. I would say that in the class of disposable batteries, they're on top of their game and have made a high quality product for the average consumer market. The ProVari, on the other hand, is in a different class of device and appeals to a different market with different needs. I don't think anyone who has owned a ProVari can argue that the quality is top notch, even if they think other devices in the same class offer "good enough" quality at lower prices. It's been my experience that quality is subjective based on the price point, design goals, and the needs of the consumer.
Features
Yes, the ProVari wins this category hands down, but what if we look at the features that we really need and not just the geeky-cool stuff the ProVari offers? If you're an average vaper, all you need is a way to get the voltage to a point where the atty/carto/clearo + juice combination tastes best and produces adequate vapor for you. For ease of use, the Twist stands out because you can easily just tweak the dial a little in one direction or another and vape to find that sweet spot. On the other hand, the geeky-cool features of the ProVari are quite useful to me and I do like knowing the current charge of my battery and the ohms of the juice delivery system attached. Both do the most important feature well, which is changing the voltage, and to me that's really what matters most.
Summary
I'll use a technology analogy since vape gear is technology and just as trendy as other tech gadgets. If I bought a MacBook Pro or a top of the line Windows PC and a Kindle Fire to use for different scenarios, I would expect each to have quality and features for it's price point and intended purpose. Each would have some of the same features, such as email, web browsing, watching YT videos, etc, but one would be a "do everything" device, and one would be a "do everything I need right now" device. I could take the Kindle just about anywhere with me with fewer concerns about damage due to the lower replacement cost, but I couldn't run a high quality video editing software on it, if that's what I wanted to do.
To me, it would make more sense if the title of this thread would have been "ProVari & eGo Twist" rather than the standard "vs" comparison as each has it's distinct place in the market and in my vape kit.