Please don't get the GTX 1050Ti as suggested above. You already had an R9 280X which is better than the 1050Ti.
A GTX 1080 would be ideal for AAA gaming, ultra details, using a 1080p/~144Hz monitor, or a 1440p/~120Hz monitor. So, if you only have a 1080p/60Hz monitor now, the GTX 1080 will be an overkill.
4K gaming requires a lot of GPU power that you might not be able to enjoy smooth frames using a single GTX 1080 without decreasing a lot of graphic details in AAA gaming. So, on a 4K monitor, the GTX 1080 will struggle in AAA games in ultra details.
I would highly recommend, for gaming purposes, to get a faster monitor on an ample resolution such as a 1440p/120Hz (preferably, with G-Sync, if your budget permits).
As to what GTX 1080 you will choose, get the cheapest one you can afford, taking into account the length of the GPU against the spatial clearance inside your chassis.
It is also preferable to get a GPU with at least dual fans for better heat dissipation on a lower noise level (compared to a single fan in ITX-sized GPUs). The looks of the GPU is your call, as we don't know what your preferences are.
All GTX 1080s, when compared amongst each other, will have the more-or-less same graphics performance because you are comparing similar chips. Yes, some GPUs of the same model have higher clock speeds than the others, but, the difference is miniscule (a few MHz only), and you can OC aftermarket GPUs anyway.
What only differentiates the models are its noise, thermal, and power consumption performance - as well as aesthetics/physical size - resulting in the differences in prices.