If you go the GPU route i would say there is only one alternative and thats Redshift. Here is a great inside from Blizzard about there Redshift pipeline. https://www.youtube.com/watch?v=24aqcFYX0LY
Arnold is a nice renderer. And the GPU update is great. Just bear in mind that some of the stuff that works in CPU mode doesn't work in GPU. From memory (hope I got it right), currently it was stuff like matte shaders, hair/fur related settings and volumetrics. Also, it won't watermark when you do a 'Render Sequence' but…