The new RadeonSI Mesa driver is a massive improvement over the previous generation drivers. My R9 290 only gets better every 6 months when I upgrade to a new Fedora release, and it's all because AMD publishes specification documents while the majority of nouveau development is still based on reverse engineering. If you care about performance, the Volcanic Islands cards and newer are all supported by AMD's proprietary AMDGPU-PRO driver which is still better than the open source ones, but I expect that gap to continue closing a lot faster than the past as the Gallium-based RadeonSI driver is a lot easier for developers to work on.

Still, comparing NVidia's current lineup to AMD isn't apples to oranges. If you want a 1070 or 1080 just get one, Vega is still months away and while we see AMD stomping in Vulkan and DX12 benchmarks compared against similarly priced cards (RX480 vs GTX 1060) they don't have a proper answer for the higher end cards until Vega is out.

Otherwise, don't let driver support deter you, the whole design behind AMDGPU-PRO is much like NVidia's proprietary drivers - it lets AMD reuse the majority of their Windows drivers to keep parity between platforms; and, again, the open source drivers only keep getting better at a rapid pace.