I built a small router using both glacern 20 mm rails and CNCRouterParts linear carriages. Both were easy to install and worked very well. I chose to use the Glacern rails for my horizontal axis because I was concerned that the horizontal axis situated below my cutter could get fouled with dust. I chose to use the CNCRouterParts carriages for the gantry and Z axes because I did not want to mount the round rails in a manner where the cutting motion would tend to pull the carriages out of the rails.
Regarding strength. Both were plenty strong for my purposes. I would imagine that you could build an 8 ft machine that could overstress both systems. But for the kinds of work that most people are doing either one should work fine.