i have had a couple of both. the difference is,
rails,
slideblock system uses alot less parts and seems to be no better, just easier to remove the block.
rails are lighter, alot of machined out areas
rails are narrower at the bottom. they will accept yamaha style slides, metrics take an odd skidoo slide that you usually have to order but is available.
you can with a grinder and about 1/2hour, make the rails accept yamaha slides
every other part on an m10 is the same or is interchangeable between the metric and std.
hope you have won something