Update with isolation_level vs. select with lock

[MCMartin]

What is the better approach to synch the update between multiple clients, when each row represents a task:

    update first taskstable set P=... where P='' Option(isolation_level = 2)

or 

    select first ID from taskstable with (UPDLOCK) where P='';
and when doing the update based on ID    update taskstable set P=... where ID = ... 

or any other suggestions?

[VolkerBarth]

Could you provide more details on what you are try to achieve (and possibly ask that at the new forum https://sqlaanswers.com)?

[MCMartin]

Thanks for the suggestion, I am waiting for my user account on the Forum