mapping implemented in functions vs in structures

I want to map entity x1 on machine m1 to y1 on m2 ...

I can see there are two choices and don't know which one
is the more idiomatic way in erlang world:

(1) functions
    m1_m2(x1) -> y1;
    m1_m2(x2) -> y2.
    m3_m6(a1) -> b1;
    m3_m6(a2) -> b2....
    
    At run time, 
    suppose A = m1, B = m2, X = x1.
            To get y1,
            I will need to form a string S ="m1_m2(x1)" 
            and evaluate it.
            btw, how do I do this ?   erl_eval:expr() looks complicated.
            
(2) structures
   {{m1_m2, [{x1, y1},
             {x2, y2}]},
    {m3_m6, [{a1, b1},
             {a2, b2}]}
             
    Given A, B, X in (1),  finding y1 becomes
    a matter of parsing the structure.
    
thanks,
HP