No Tomlinson is a stud and gets you a lot of yards while Johnson isn't bad and is a good RB I don't think its worth it because Jake Westbrook hasn't had the best year, he gives up a lot of runs and not striking out enough guys.
Let me toss in a contrarian opinion here and say that you should seriously consider it. SD has a BIATCH of a schedule this year that could prohibit LT2 from being the blowout #1 the many were expecting. He could still end up #1, but it will be a close race barring any serious injuries to those involved.