They're essentially the same camera; the 517 usually comes with lesser shutters and lenses, but that's about it. I'd aim for (at least) a Pronto shutter, but the Prontor in any configuration - plain, S, SV, SVS - is more desirable; the usual three-element Novar Anastigmat lens can be had in f/6,3, f/4,5 and f/3,5 variants. The fastest isn't the best - in fact, the f/6,3 is quite a solid performer. There are even some older Nettar models (516/16 and 516/2) that can be had with Tessar lenses, but they are usually pretty expensive - for that kind of money, I'd head straight for a Super Ikonta instead. My 518/16 has a Prontor-SV (1"-1/300", flash socket, self timer) and a 75mm f/3,5 Novar Anastigmat; stopped down to f/8, it's quite a sharp lens. The f/6,3 variant is just as sharp wide open - but of course, you lose 1 2/3 of a stop you could use in a pinch.
There's actually an alternative as well in the same price bracket - the Voigtländer Perkeo I; it's smaller, more elegant and even better built. But the Vaskar 75mm f/4.5 lens isn't as sharp as the Novar Anastigmat even stopped down, and it only has a Pronto shutter (1/25", 1/50", 1/100", 1/200") - but it's usually quite sufficient. The Nettar 517/16 usually comes with the Pronto shutter and the Novar Anastigmat 75mm f/4.5 - so that's the camera I'd pick over the Perkeo I. But here's the thing: The Perkeo II is a different matter, though - it sports a Color Skopar (=Tessar!) 80mm f/3,5 lens; its film handling is fiddly, but more advanced (it has a counter!), and it comes with a Compur Rapid shutter (1"-1/250") that's more precise than the Prontor (but harder to service if it's stuck or slow). That camera can sometimes be found quite cheaply; it's the best camera in the below-$100 range at the moment - though things can change quite rapidly in that market. That said, I still prefer the Nettar in the hand - it's a bit bigger, but the "drawbridge" configuration makes for a more comfortable hold.
btw. I'd also think about an additional rangefinder; they can be had for less than $50 - Voigtländer ones are most common and can be attached to the cold shoe on the 518/16, but I picked up an old handheld Leica one for $20 that I actually prefer.
Last year (2018!) I finally found my dream folder, though: a Super Ikonta 531/16 with Tessar lens and coupled(!) rangefinder. It's hardly bigger than the 518/16, but a much, much better camera. I got mine for $130 in full working condition (I had it checked regardless to make completely sure). So, in the end, it all depends on your patience and determination. If it's just meant for playing around, a Nettar 518/16 or 517/16 will serve you fine. But for little more money, much more interesting cameras (later Ikontas and Super Ikontas - i.e. uncoupled and coupled rangefinders - and the Perkeo II) can be had. It's your choice.
M.