L.,
my son is 28 months, and is in the 97% percentile for both height and weight. i can't remember exactly but at his two year old check up i know he was over 35 lbs. he looks like more of a three or four year old! he is also very advanced in vocabulary and social skills, etc. point being, i will not trust my little wiggle worm to those restaurant booster seats. we tried one time, he was probably 18-20 months old, and i did nothing but "rescue" him from tipping over, "rescue" the salt and drinks, etc, the entire time. it didn't last long! 1. he's not secure, and 2. he can reach everything! so i haven't put him in one since. my criteria will be when he is mature enough to sit still somewhat, and not destroy everything in reach. don't know yet when that will be! and it might be unrealistic. but for now i stick to the highchairs. don't worry about height/weight restrictions. as long as he's not breaking the seat i think he's fine!
oh and at home we don't use the highchair as much - we have a corner kitchen nook with the L-shaped bench, and he sits on that just fine. kids can reach the table fine, it seems, when food is on the line! (at least my little tank can!) lol. do what works for you.