GSG and Sig Sauer 1911s are manufactured by GSG. Other than different choices in finish and grips, the guns are the same. Sig may offer a better warranty.
Now the important thing is you and your son ought to plan lots of time having lots of fun. How about a range report?
The finish on the rails may need to be smoothed down some and may take about 200 - 300 rounds to do that. Using high velocity, round-nose, copper washed/plated rounds will make that break in period easier.