I've read somewhere that autopano giga doesnt use any GPU for increasing the speed of rendering. I have a i7 and a xeon but both need about 3-4 minutes per pano which is quite looong. Are you planning to use opencl in future (in that case I will keep my 3x1070) or not (sell 2x 1070 and get a beefier CPU)
If so, which is the best cpu to go for? What is the key factor? CPU speed? Number of cores? CPU cache?
This is a feature we are thinking about but I cannot confirm (at this time) if we will add it in a later version. Autopano will use all the cores available but does not support multi-CPU. rendering time also depends on the images (JPG, TIFF, RAW), their resolution, their number, the corrections you added, numbers of CPUs and so on. A Xeon is a perfect choice. CPU speed and number of cores are important!