When it comes down to basics, Luxus is a far more hands-on converter than Reality. While in Reality 2.5 you have access to basic material settings, in Luxus you can control literally everything and have access to every material Lux does. The upside to this is that you can render very complex scenes using Luxus without ever needing to manually edit the Luxus files directly, the downside to this is it does require an understanding of how Lux materials work. By contrast, Reality 2.5 makes it far easier to convert materials, but does so at an upper level and by doing so it denies you access to the underlying mechanics and fine tuning.
Luxus, like Reality, will automatically convert materials based on certain criteria if they haven’t been given specific Luxrender materials. This usually results in a glossy material, much like Reality 2.5 does for materials. I’ve found Luxus’ material conversions very similar to Reality, but Reality does have an advantage when it comes to camera options and IBLs so that things like Depth of Field are easier to execute.
As to the source of your crashes, GPU acceleration is the most likely cause. Luxrender still has significant issues when trying to use hybrid rendering and even the 1.3 dev release hasn’t fixed this problem. I have an OpenCL compatible graphics card (I can render just fine in SmallLuxGPU [SLG]) but it refuses to render in hybrid mode due to a bug which hasn’t been fixed. For some scenes however, there is a fix. Enter the following line into the render settings of your LXS file exported by Reality 2.5.
“integer opencl.gpu.workgroup.size” 
That may enable you to render using Hybrid rendering, but it’s not a guarantee.