I knew my reply would get a few responses.
I used to use the WO in SH3, since I was lousy in manual TDC, but it was abit of a cheat since he gives you a perfect solution, sort of like auto TDC, but again it is a personal choice.
I actually find manual TDC easier in SH4, in part because the lack of a working chronometer forced me to learn proper RL techniques, but also because the US TDC as implemented in the game is much more sophisticated and flexible than the SH3 TDC.
In SH3, as soon as you had a solution, you had to fire right away, since the solution is only accurate at that instant. The SH4 U.S. TDC constantly updates the solution so you can wait and still have a good solution, it also allows you to check if your solution is good, something which you cannot do in SH3.
Furthermore, in SH4, you can easily shift the trajectory of individual torpedoes 1 or more degree left or right on the fly to setup a spread. All in all, much more user friendly.
In SH3, I was a lousy shot while in SH4, I'm hitting 80-90% of the time.