It's not the flush mechanism so much as the type of cistern coupling that makes a difference. On a conventional high or low level cistern you've go a flush pipe between the tank and the pan, so you can normally get at the nut underneath the tank that holds the mechanism in, without disturbing things too much. With a close-coupled cistern, the tank bolts directly to the pan, so you have to take the tank off to replace the flush.When it comes to the actual mechanism, you can either go for the traditional syphon or the more modern valve type.I always use the latter these days as they take much less force to operate and they're easier to set up the dual-flush levels on. The other big plus is that you can get ones where the valve body clicks onto a bayonet fitting in the tank so, if it develops a fault, you can whip it off and service or replace it without touching the plumbing.The ones I've used recently, Fluidmaster, come in either a push-button or lever-action version. The downside of the lever-action one is the lever tends to rotate in its hole because people assume it's a syphon valve and so give it the traditional amount of welly!