from my understanding, the 16" give a slightly lighter weight bow, more forgiveness in form, from longer working limbs. the 18" heavier bow weight and just a little more "whip" caused by shorter limbs. that is given that the NTN of the bow is the same. i have now built 2 with the 16" and one with 18". one of the 16" is the same weight as the 18", both 68" hill styles, the one with the 16" riser averaged 9fps faster, but that was over 25 shots for each bow. the numbers varied alot due to the fact that i shot by hand rather than using a shooter.
I'm probably completely wrong in this. some else will have a better understanding and definatly more experience. i only have 7 glass bows under my belt. me and my brother were curious so we played around. don't remember what the bows weight were, but they were only a few ounces differences.