3ds max rendering comparison

We will be purchasing some Renderboxx 10600 units, looking for price performance comparison between the DUAL XEONs
DUAL XEON E5-2620 2.0GHz, 15MB cache, 7.20 QPI (Six-Core)
DUAL XEON E5-2630 2.3GHz, 15MB cache, 7.20 QPI (Six-Core) [+$516]
DUAL XEON E5-2640 2.5GHz, 15MB cache, 7.20 QPI (Six-Core) [+$1096]
DUAL XEON E5-2650 2.0GHz, 20MB cache, 8.00 QPI (Eight-Core) [+$1870]
DUAL XEON E5-2670 2.6GHz, 20MB cache, 8.00 QPI (Eight-Core) [+$2902]
DUAL XEON E5-2680 2.7GHz, 20MB cache, 8.00 QPI (Eight-Core) [+$3289]
DUAL XEON E5-2690 2.9GHz, 20MB cache, 8.00 QPI (Eight-Core) [+$4063]

Any advice is welcome...
Thanks
3ds max 2012 pure rendering