I think it really depends on your application.
The C6100's are great, for cheap cloud servers. They have NO convenience, limitd in expansion, limited in RAM, no SATA/SAS 6gbps, sharp edges and not user friendly for entry level techs.
The R710's are great chassis as well. They are easy to work on, have 6gbps, lots of expansion slots, more 2.5" bays, more ram bays, actual hotswap power supplies etc.
It really depends on your application. If you're looking to bring up a bunch of nodes, fast, in a cloud environment where hardware is expandable, C6100. If you are going to drop this in for a customer, he may have to work on this himself, expects servicability on site, quickly, and full dell support ... R710.
My $0.02.