Pretty much the only retail outlets you're going to find will be the mall kiosks, and those are primarily populated by Smoking Everywhere shops. The conventional wisdom on these boards is to stear clear . . . expensive, expensive, expensive.
You're basically going to be buying online, but no worries . . . easy enough to do.
I, too, think you're right on the money with a Screwdriver or a Dura-C (branded version of the 510). Manual batteries are the bomb.
Both provide good, strong vapes. As wv2win explained, the batteries are a HUGE difference. Depending on whether you're a light vapor or a heavy vapor, you'll find anywhere from 1 1/2 hours to 4 hours on a battery charge for the 510. With a SD, though, you'll easily get 4 hours, and you'll most likely get 6-8. Plus, the SD batteries are indeed cheaper.
With that in mind, why would anyone go with a 510 instead of a SD? Well, first, the 510 is substantially cheaper. You can pick up a 510 from
www.widowsbeadwork.com for $50 (s/h is free). That's a heck of a lot cheaper than the current $150+ (depending on the exchange rate) for the Screwdriver being shipped from the UK. (I understand Totally Wicked will be carrying the SD soon, but even so, it's pricey.)
Now, *I* think that ultimately, I'll save money with the SD because the batteries are so cheap. Likewise, the 901 atomizers the SD uses are cheaper than the 510 atomizers.
The other reason someone might want to go with a 510 as opposed to a SD is because the 510 is smaller and looks vaguely analog-like in terms of its size and feel. Some folks simply feel more comfortable vaping with a device that looks like a tobacco cigarette or a pen, and making the transition to something more reminiscent of, well, a screwdriver, may be kind of hard.
Of course, after you've been vaping for awhile, you'll pretty much get to the point where you don't care at all what it looks like . . . it really does become function over form.
Anyway, you can't go wrong with either the SD or the 510, imo.