Neither of them will help directly with CPU usage while a scheduled task is running.

Request mode will check to see if the scheduled task needs to be ran on each request, so if you have low traffic it might miss something until the next time you get a request.

If you have very high traffic then it may be checking too often in Request mode, but it won't run the task until it is time for it.

In timer mode, it is a little more accurate, but it has a background thread running which may be interuppted with application restarts, and there is no context of the web application when running on a background thread.

Personally, I would recommend turning off all scheduled tasks, and then coming in and performing the maintenance that the tasks are providing manually.  That's just me though, others may not have a problem with batch tasks running from a web environment.

Thanks, I'm glad to help, especially for those that appreciate it.

Yes, on my sites I disable the scheduler (set it to none, and disable all the tasks).

For UsersOnline, the one in host settings will stop it from collecting the information and writing to the table, the scheduler task for UsersOnline "cleans up" that table by deleting old records.
You would need to delete old records manually if you have the UsersOnline module enabled in host settings and the task disabled in the scheduler.

 
Personally, I don't use the UsersOnline module either.  It causes a lot of database activity for what amounts to a "vanity" control in my opinion.  Activity on your site can be measured in many different ways that don't affect performance like this.  I'm not saying there is anything wrong if you like to use that module, but you should realize that it will affect performance.