I have the Samyang (Rokinon) 14mm on 6D. This is a great lens, with excellent sharpness and very low price. However, you have to keep in mind that 14mm on FF is VERY wide! I am very happy to have it and I have never regretted buying it, but I can't say it's my most used lens since it is often just too wide. It really depends on how wide you want to go; the 16-28mm FOV is much more practical, but the 14mm is much more unique! I have a 28-75mm walk-around lens. Just wondering, does it mean that you don't really use this lens, except perhaps for landscapes? Are there any other time you use it?
Also, do you have other wide angle lens? For instance, if you already have a 24mm, I would recommend more easily the 14mm than if your widest lens is a 50mm!
Since it's so wide, it's not as versatile as a more "traditional" focal length. I use it not only for landspace, but also for group photos (especially with big groups and limited space), bands, "over-the-top" portrait, etc. For me, it is a speciality lens, a bit like a rectilinear fish-eye; it can't be used in any situation, but when it can, it's astonishing. That's the reason why I bought a used 28mm a month after I got a 6D; the gap between 14mm and 35mm was too big. I guess it really depends on the person.
IMO, you should be okay with 14mm and 28-75mm; 28mm on FF is pretty wide, and when it's not enough, there's no doubt that 14mm is going to be! You lose the versatile of the ultra-wide zoom, but gain the widest lens you can imagine (excluding fish-eyes). Not to mention that it is literally half the price! Moreover, if you have decided to upgrade to FF to gain wider FOV, the 14mm is really the thing to have. Of course, the best would be to have both, but if you are like me, it's going to have to wait a bit.
I hope this help.