So I have both samyang 35mm f2.8 and 18mm f2.8. Worth saying that I actually like Samyang lenses and tend to choose them over others - the price advantage usually doesn't do any harm at all either, but that's not the sole driver.
Before buying the Sammy 18 I considered the Tamron 20. Price isn't especially different, but the Tammy is a much bulkier lens despite having 2mm less wideness. It's key selling point - very close focus - isn't attractive *to me* and because the Sammy is tiny, lightweight and was available as a UK purchase for just a few quid more than E-Infin pricing I grabbed one. So far it's the sharpest super-wide angle I've ever owned, and I'm happy with it, as you can tell. Flare resistance is good for a WA and if stopped down it's sharp right into the corners (I don't usually use a WA wide open).
Between 2 temples 4 by
Toni Ertl, on Flickr
Between 2 temples 1 by
Toni Ertl, on Flickr
The Sammy 35 f2.8 is a real favourite for street work because it makes the A7 look like a chunky compact, and using silent shutter and composing on the flipped screen like a WLF it lets me take pictures without being obvious. I used it a lot on the street in Morocco, and barely raised an eyebrow. Stopped down it's sharp corner to corner. If you use it wide open then the corners can look a little ugly - bokeh is NOT a strong point either - but the tiny size and feather weight make it a winner for me.
Bike and barrowman mogador by
Toni Ertl, on Flickr
Market-06000 by
Toni Ertl, on Flickr
The thing that would put me off the Tammy 17-28 is that it's just the wrong range for a WA zoom. 17mm at the wide end tells me it should go to 35-40mm at the tele end and be a true all-rounder for street use. Alternatively with 28mm at the tele end it should go down to 12-14mm at the wide end, but it isn't a really ultra-wide WA. Granted the f2.8 aperture is fast, but a fast aperture isn't especially important for a UW in my opinion, and it's too dear just to have as an 'in case' lens. Others will feel differently.