Sloping top tube?

I was told that by the builder that the choice of Aluminum (Easton), would be cheaper than steel (Reynolds, Columbus or Vitus GT)...now, I'm aware of most diferences (pros&cons) of Aluminum vs Steel, but in case of choosing Aluminum, would that be compromising too much? (Rigidity-wise)

Depends on what you want, ride wise. Alu frames get a bad rap, because most of the low end frame-sets are alu. Doesnt mean you cant make great frame out of it. So its no comprimise at all, just personal choice

I like steel, but aspire to titanium ;)

Is the builder giving imput on tubing dimensions?

With the right tubing selection you can tune the bike to your weight and power.