Datetime vs. Timestamp

[Michael Corr]

Hi,
I've been reading through the manual and some of the user posts trying to figure out the advantages of using DATETIME vs. TIMESTAMP. I have a simple requirement of updating a time field in my table everytime a new row is inserted or existing row is updated. Can someone please tell me which is better:

create table t1 (col1 int not null, ts timestamp default current_timestamp on update current_timestamp not null)
insert into t1 (col1) values (1);
update t1 set col1=2;

-or-

create table t2 (col1 int not null, ts datetime not null);
insert into t2 values (1, now());
update t2 set col1=2, ts=now();

Both methods seem to have the same effect. Is calling the now() function slower than using current_timestamp ?

thanks,
Michael