At similar apertures, the 15 Limited is sharper than the DA 14, and way smaller. Since most people use this wide of a lens for landscapes the fast aperture is less important.
For your stated purpose of using that wide max aperture, neither the DA 14 nor the Tamron 14 is particularly sharp wide open.
You'll get much better results from something like the Tamron 17-50/2.8, Sigma 17-50/2.8 or DA* 16-50/2.8. Unless you're right up in the band's faces 14-15mm is probably going to be too wide anyway.
Really the ideal setup for what you desire is an FF body and a 24/1.4. Or for not quite as ridiculous money the Nikon 28/1.8.