difference between beretta 3901/391 20 gauge

what is the difference b/w these guns?? is it because one is a synthetic stock? my son is 10 and in 4-h shooting sports (trap/skeet). we were told the gun they recommend is beretta youth 391/20 gauge, but can't afford $1000, at least not until we are sure he is going to continue shooting.
if anyone has any information or can give me any suggestions on a different gun that is not as pricey as 391 would appreciate it very much