I don't know ROBO SNOWMAN, I kind of like your first idea better.
A good diorama should tell a story. A simple story is easier for the viewer to understand.
If a dio is complicated on the other hand, you risk confusing the audience as to what it's really about.
With your first idea, I think viewers will get it right away. A sniper and his spotter take out an enemy.
With the second idea, non-Gundam fans may be confused by what's going on. Who is the sniper shooting at? Why is he shooting if that other guy has it under control? If he's not shooting, why is he there?
You should also try to keep you display as simple as possible for other reasons. Hypothetically, let's suppose your models are really kick a**, but the trees in your dio are substandard. I guarantee you that viewers will fixate on those bad trees. The more elements you include in a base, the more chances you have to mess something up.
Additionally, expansive dioramas are expensive, both in terms of money and time. You could add six trees behind your model, but would just one or two convey the same impression?
Large dios are also harder to store around your house. At my place, I have two bookcases and a curio cabinet just to store all my completed models. Honestly, I have no idea where I'm going to put the stuff I'm working on now...
Just some stuff to think about. Feel free to ignore my ramblings.