My IT provider is convincing me there is no difference between a 2Rx4 and a 2Rx8 RAM. A little digging around informed me that x4 means 4 bits on a chip vs x8 means 8 bits. Which is good, but what difference does that make?
2R == 2 RANKS, this is the number of chip selects each DIMM module has.
x4 == data-bus width of each DRAM (chip)
DDR* memory bus width is 64-bits wide.
So a single 1Rx8 (non-ecc, unbuffered) DIMM will have 8 DRAMS (chips) ... 1Rx4 will have 16 1Rx16 will have 4
Ranks, on the other hand, are 64-bit arrays that share the bus. Only one rank can have the bus at a time, the chip select line is the ranks way of knowing that it is that ranks turn to have the bus.
So a 2Rx8 will have 16 chips.
First and foremost, the difference is that they are completely incompatible with each other for use in most server boards. If you're replacing an existing memory module you must either match the given designation perfectly or replace the whole bank with the new type. If you're evaluating a completely new configuration, then see Tom Shaw's answer, and when checking compatibility, the "1x" means "Single Rank" and "2x" means "Double Rank" if the system specifications call for one or the other specifically.
There's not enough information to say which is better. When looking at RAM you probably care about a number of things:
- Density (bytes per slot)
- Performance (RAM timings)
- Reliability (quality, ECC vs non-ECC)
- Compatibility (will it work?)
- Supportability (vendor requirements)
- Power usage (heat dissipation)
The 2Rx4 and 2Rx8 DIMMs may well give you different characteristics for many reasons, but you'd need to ask your vendor about those characteristics directly.
If it is the right module (example: PC4-12800) then it should fit the slot and perform as well as the relevant standard (example: DDR4-1600K). The matter of integration within components versus on the board of the DIMM is usually not a concern to most users. A DIMM with fewer chips might use a little less power. If power is the constraint then you might solve it by changing DIMMs but that would be a low quality solution.
That said, adhere to the instructions of the motherboard manufacturer. The sizes and placement (matched sizes in the correct slots) of the DIMM modules can have a large effect on the throughput of the system even when the quantity of memory is more than adequate. (Note: I am not referring to the size of components used to assemble the ranks on the DIMM. I am referring to the DIMM size.)