From LuxRender Wiki
In order to create nice renderings, it is not strictly necessary to manually set all render settings; usually one of the presets will suffice. Render presets can be selected in a dropdown menu at the top part of LuxBlend.
Settings for samplers, integrators, volume integrators and filters can be set in LuxBlend's Render tab. For an explanation of their function within the rendering process, please refer to LuxRender_Render_settings.
Render Presets and Their Uses
Preset 0 (Preview)
Typically only good for checking of basic scene geometry. Reflection, Refraction, Transmission and even Specular color will not work under this setting. Pretty much only diffuse colors will work. (In LuxBlend 0.7-dev there is also Global Illumination preview preset besides Direct Lighting -- that one is more useful, it's based on the same settings which are used for material previews.)
Preset 1 (MLT/Bidir Final)
The most advanced and versatile sampler/integrator combination, useful for almost all situations. It gives the best results in interior scenes, but works well on exteriors too as for example sun caustics will converge more quickly than when using other presets.
Preset 2 (MLT/Path Final)
Path tracing is a less sophisticated rendering method than Bidirectional Path Tracing. It can be useful in exterior scenes, although even there in some cases it can be slower than preset one. However, Path is the only option if you use volumetrics (with boundvolume material) since bidir does not currently support them.
Presets 5 and 6 (Progressive Bidir and Path)
Same as 1 and 2, but uses low discrepancy sampler instead of Metropolis Light Transport (MLT). Useful only with the simplest scenes as low discrepancy usually takes much more time to converge, however with simple light scenarios and in the absence of caustics could be faster.
Presets 8 and 9 (Bucket Bidir and Path)
Same as 1 and 2, but uses Hilbert low discrepancy sampler. It processes image buffer in buckets (blocks) which significantly reduces RAM usage. Therefore, it is useful in scenarios where using other samplers would use so much RAM that the computer would start swapping.