I think order in which alternative are tried only improves the no. of times parser has to backtrack.
Suppose
S->aAb|aB
A->c|d
B->ddc
Check for w=addc.
Parser will choose S->aAb and undergoes backtracking 2 times. If it can choose another alternative i.e S->aB there will be no backtracking.
As far as talking about languages it will be the same because ultimately parser will generate the language whether it has generated it with backtracking or without backtracking.