I always use count Vs using num_rows. My site uses a lot of queries, and the load on the server is fairly light given the size of my site.

Using an asterisk (*) wildcards count() to consider all fields. It does add a very small (not even noticeable) lag in the query process. However, if you only need to get count totals from a specific field, you can use just that field_name and it serves the same purpose.

I guess it all comes down to personal preferences. Some programmers like to code it one way, and others another way.