There is no difference between the first class service/seats on the two flights. One has 4 rows of first class, the other 7... it is otherwise identical.
There are actually two flights ouf of IAH each day.. one is on the 767 and one is on the 764.
Coming from Boston, I'd rather take the EWR flight. It seems to make the trip go by quicker when you have the long leg all at once...