Installation of the coolers is easy with Antec/Corsair using an identical process but Cooler Master go for their own design which works well. In fact the use of these new brackets is a real bonus on the Seidon which thanks to thicker metal and a better backplate feels like a higher quality unit. That said Cooler Masters LED on top is a bit of a non-event... it's not bright enough or specific enough (e.g. a logo) to make much difference.
For other differences the tubing used on Antec and Corsair's models is easier to work with and users will have to decide if the dual connector (Corsair/CM) or single connector (Antec) design is more to their tastes.
When it comes to value, those concerned about noise at full RPM may just get away with the 620, and will be fine with the H55 as is. Seidon users will need to buy an alternative fan though which adds a little extra cost. Other than that they all price around the same level so the choice should be down to how the various design tweaks appeal to each consumer.
Corsair H55 Liquid CPU Cooler
Cooler Master Seidon 120M Liquid CPU Cooler
Antec Kühler H2O 620 v4