HPL2 vs HPL1 vs other indie engines

[Argoon]

HPL 2 does support sky boxes and nothing prevents you of making large levels with terrain (no voxel terrain but that is not a problem theres a 3r party tool for that like terragen), trees (no LOD system afaik) and all, one thing you just need to have in mind is that your models need to be comprised of various submeshs for the automatic culling system to work as it should.

About the HPL1 and the other engines that you said, HPL1 is free and you have access to the source code only lack of coding skills will prevent you of using and improving this engine. C4 is VERY good but it costs money (+-100 € for the Basic edition) you have access to the source code is a very well written c++ engine and you have one year of free updates and developer support. Torque i don't know. Unity tools and easy of use is fantastic but in the free version you lack very important things that exist on the pro version and on other free render engines like OGRE and HPL1, one of them is real time shadows and Unity even on the Pro version don't give you access to the engine source code.