March 18, 2014, 11:11:34 pm
The problem I guess you face is:- better off with more VRAM in the 770 or more performance in the 780.
If you look at the submitted Benchmark scores no 770's, so closest is a GTX 670, there is the difference for Lumion of about 30%.
Comparison of the NVIDIA specs from their web site shows a significant difference in all main specs.
If the GTX 770 had 3GB the choice would be clear, so the thing to consider is will you gain from the extra 1GB of VRAM. That would depend on the known or expected type of scenes to be produced with Lumion. If they are going to be 'more' complex and detailed, the scene may require more support from the VRAM. However, there are simple rendering optimisations to reduce texturing costs that can be employed to offset pushing the limits of the GPU accessible VRAM.
Probably 3GB is adequate for many scenes. The problem we have is that we have no available measures of what scenes take in terms of VRAM, and every scene has such unique demands that making more precise guides is not really possible (but getting some data would be nice).
From a personal viewpoint, I think I would opt for the GPU that will give better rendering times, thus saving time and allowing more tasks to be done. The GTX 780 has about 700 extra cores, and better throughput, so could cope with processing and pushing the data around a lot better than the 770. The 770 is more of a pricing point solution, and for professional based requirements platforms, the higher the specs (both VRAM and performance), the better off a user should be (all other things being equal).