Adrianot,
I am not certain why Spare Parts said that. If you order dimm part number 317-6142 it is compatible in either system.
In order to limit any issues and support the large dimms, you need to update the BIOS and ESM/Drac to current.
R610 BIOS - www.dell.com/.../poweredge-r610
R610 ESM - www.dell.com/.../poweredge-r610
R710 BIOS - www.dell.com/.../poweredge-r710
R710 ESM - www.dell.com/.../poweredge-r710
Let me know how it goes.