Which update action quicker?

<div class="moz-text-flowed" lang="x-western" style="font-size: 18px;"><small>Hello list, <br /><br /> For a big table
withmore than 1,000,000 records, may I know which update is quicker please? <br /><br /> (1) update t1 <br />       set
c1= a.c1 <br />       from a <br />       where pk and <br />                  t1.c1       <> a.c1; <br />
 ......<br />       update t1 <br />       set c_N = a.c_N <br />       from a <br />       where pk and <br />
                t1.c_N       <> a.c_N; <br /><br /><br /> (2)  update t1 <br />       set c1 = a.c1 , <br />
           c2  = a.c2, <br />             ... <br />             c_N = a.c_N <br />      from a <br />      where pk
AND<br />                (  t1.c1 <> a.c1 OR t1.c2 <> a.c2..... t1.c_N <> a.c_N) <br /><br /><br />
Orother quicker way for update action? <br /><br /> Thank you <br /> Emi </small><br /></div>