which Update quicker

Hello list,

For a big table with more than 1,000,000 records, may I know which
update is quicker please?

(1) update t1
       set c1 = a.c1
       from a
       where pk and
                  t1.c1       <> a.c1;
  ......
       update t1
       set c_N = a.c_N
       from a
       where pk and
                  t1.c_N       <> a.c_N;


(2)  update t1
       set c1 = a.c1 ,
             c2  = a.c2,
             ...
             c_N = a.c_N
      from a
      where pk AND
                (  t1.c1 <> a.c1 OR t1.c2 <> a.c2..... t1.c_N <> a.c_N)


Or other quicker way for update action?

Thank you
Emi