Usually it is preferred to write
var A = B + C;
because then the expression “B + C” is substituted for A; the solver does not see the variable A or the constraint “A = B + C”. However, there can be exceptions where the number of “defined variables” like A is very large. So if you are finding that AMPL is taking an especially long time to generate the problem, you may want to try the alternative,
var A;
subject to Adefn: A = B + C;
to see whether it is faster.