The R510 is limited to 128GB of RAM as it has only 8 DIMM slots and only supports two out of three available memory channels on the CPU. The R710 supports up to 192GB (18 DIMM slots) and fully supports all three memory channels per CPU. Even if larger RAM configs are not an issue for you the R710 supports 50% more memory bandwidth because it uses that extra memory controller channel.
The R710 has four onboard Broadcom NICs, the R510 has two.
The R710 can support a PCIe x16 slot and a maximum of 4 PCIe devices, the R510 supports at most 3 and can’t handle x16.
The R710 supports hot pluggable redundant PSU’s, the R510 does not have hot pluggable PSU’s. Edited to correct: Apparently this is no longer true, and may never have been. The 750W and 1100W PSU’s options are hot pluggable.
The R710 supports a wider variety of rack types with standard Dell accessories.
I’ve used the R710 a lot – great system and of the differences outlined above the additional memory bandwidth, hot pluggable PSU option and number of onboard NICs were the main reasons why I’d select the R710 over the R510 for general purpose server situations, in my case these were almost all used as VMware ESXi hosts.
I’ve installed the R510 a couple of times but they were always nodes in clusters that needed lots of onboard storage and where individual node resilience and performance weren’t especially important.