There is currently no official plugin for Blender. There is Octane renderer standalone usable with Blender, thanks to some good exporter scripts gently made by some Octane/blender users.
For the choice, it is a matter of taste and needs. Octane is more stable than Blender/Cycles for what I have tested, and it is faster and easier to setup materials in Octane.
However, Blender/Cycles may be more usable for animations. Export time is currently a problem on big scenes for animations. So a native internal render will skip this long process. This said, currently the rendering speed of Cycles is slower... for what I know, if I am not wrong.
French Blender user - CPU : intel Quad QX9650 at 3GHz - 8GB of RAM - Windows 7 Pro 64 bits. Display GPU : GeForce GTX 480 (2 Samsung 2443BW-1920x1600 monitors). External GPUs : two EVGA GTX 580 3GB in a Cubix GPU-Xpander Pro 2. NVidia Driver : 368.22.