I tried not the same scenes, but comparable ones in unity, 3dgs, and esenthel, each made due to my concept world.
yes, unity is definitely fast and easy, if you pay $1500 for it! the free is not okay for large scenes as it does not offer lod, dynamic shadows, and shaders for nice water, no model editor. and it "cheats" a bit, it uses static lightmap shadows for vegetation, and offers only billboard vegetation lod, which is sometimes not enough quality. by the way, its programming is really good, and the editor is okay only you have some 3d art experience. but it falls out a bit from the comparison, because of the high price of the pro.
in esenthel, you get for free or for $100 if in a commercial project: fast and really high quality pssm shadows, gpu bone animation, streaming terrain with lod like in UDK for really huge-huge worlds, more advanced model lod (several models and a billboard in the end, can be created in its model editor), grass is also handled fine but not optimized as in unity just clipped (but leaf normals can be set in its model editor which is very important for plants). it offers DX10+ rendering what I found much faster than the DX9c, and you can compile as x64 or x86 in Visual Studio, but no instancing, so a bit slower than unity, but beside higher quality. it requires more programming knowledge because it uses C++ or its own scritp language which is a bit more lightweight but still not as easy as lite-c or unity-script. importing 3d assets is just smooth supporting many formats. the world editor is also fine, you can choose or define new object classes and properties, it features a built in navmesh pathfinder. for open world project it is definitely a good choice.
in case of 3dgs you need the commercial licence for $200 to really get something, the free is okay for toying similarly to unity free. imo its scripting is the easiest, probably the best 3d engine to start with, but its editors are outdated, importing 3d assets can be problematic, you cannot make a nice terrain in MED or WED alone, you can't see anything in real time. my original concept was to make my own game editor to test my game AI on design time, so it was not a drawback for me. its outer lod system is fine, but you get no other optimizations like terrain lod or instncing, only in the pro which is over $800. it is possible to get good performance and acceptable quality if you use pssm shadows (its quality is not the best at all), but it can be a good solution to combine it with static lightmap. it still features vertex animation, what I found good if lod is applied strongly, the cpu bone animation is slow, the gpu is pro only. so this engine is a big compromise, you have to do a lot of scripting to achieve things what you get in other engines by default, but good for a one person hobbyist development.
recently I'm playing with UDK, but cannot say anything yet. it's complex as hell.
