So now is my question: if a tradeoff must be made, what is more
important, to have an antenna on the clear or to have the best ground
radials system ?

better to have the radiating part of the antenna clear. The ground
radials, as long as they are numerous, are less critical


A second question: is it possible to estimate the effect of ground
symmetry and of buildings presence on the antenna radiation pattern
using antenna simulation software like eznec ?


Yes.. to a certain degree

What you can do is put in wires where the building is (like a wireframe
drawing), and then see if there's very much current in those wires, when
the antenna is excited. Small current = little effect.

You can get fancier and try and model the building as a grid of wires
which are very resistive, but I'd start with the simple wireframe and
see what it gives you.

What you're really getting isn't a precise model, but more of a
qualitative assessment of whether there is a problem.