The RM (2019) is a cheaper version of the RMx (2018). Based off the same platform but the RM 2019 is designed to meet new energy efficiency targets that system integrators will need to meet for their systems in the state of Cancer (may cause California). It does this with a newer LLC resonant controller which boosts efficiency at low loads. Since it's targeted at system integrators it's also had a few cost cutting measures to keep the price lower. Cheaper fan, cheaper hold up capacitor, cheaper cables (without inline caps).
The RM650 uses an upgraded platform, provided by the same manufacturer (CWT) that made the RM650x. There several essential differences between those two platforms, as you can see in the table above. The areas where the RM650x wins are the higher quality Japanese caps and the NR135L fan. On the other hand, the RM650 uses a new resonant controller, allowing for higher efficiency under light loads, and it is equipped with an electromagnetic relay which enhances its inrush current protection. On top of that, it uses a micro-controller in the fan control circuit so Corsair can easily tune the fan's speed and provide the ideal fan speed profile.
The Tom's Hardware review linked also has a side by side comparison of the components used in both the RM 2019 and RMx. Though from the components listed (and lack of relay bypass for the thermistor) that appears to be comparing to the older RMx not the 2018 version.
Both are fine and neither will blow up your system, but the RMx is the better of the two if you aren't required to meet those low load efficiency targets.
9 hours ago, MiinD said:
yeah but this is for a psu the x in a psu wount make a difference they have the same watt and the same rating so its really just basd of price.
There's more to a power supply than how many watts it's rated to deliver and what its efficiency rating is. There is a difference between the two units and they did not just scratch off the x from the label and sell it as the RM (2019). This also wouldn't even make sense to do financially since the RM (2019) MSRP is $10 lower than the RMx (2018) so by your reasoning they would be releasing the same power supply at a cheaper price when they could just keep selling it under the RMx (2018) branding at a higher price.