I have the Metz flash but have no experience of the Canon 550EX. However, looking at the specifications, both offer very similar functions, power levels and the ability to act as wireless (optical) master and slave flashes. The Metz has an additional small forward facing flash, the purpose of this is to add catch lights and lighten shadows for portraits when the main reflector is positioned to bounce off the wall or ceiling. If you are only looking to keep one flash, then I suggest you establish which interface you find easiest to navigate. But why not keep both, use one as the master on the camera hotshoe and the second on a light stand or tripod as a slave, the camera and flashes communicate using the Canon ETTL II protocol with optical light pre flashes. You can vary the power levels for each flash as ratios for different creative effects. I suggest you Google 'Strobist 101' for detailed guidance and inspiration. Add 3 Yongnuo YN622C transceivers for about £60 and TTL (through the lens) metering information is transmitted by radio signals which increases the range and works well in brighter light. Both flashes could then be mounted off the camera. Lastly, you could sell one of the flashes and replace it so you have two flashes from one brand, which saves learning two different interfaces, they look to cost about the same secondhand despite the age of the 550EX.
Note that the flash (or transceiver) has to be mounted onto the 5D2 and turned on before you can access the flash set up in the camera menu pages.
