RFB emphasizes historical accuracy. Ships tend to behave as they really did. The downside is that this makes them predictable, so once you figure them out it becomes easier than it really was.
TMO empasizes accurate gameplay. Destroyers tend to be better than they were, which forces the player to be more careful. This means that some complain about it being harder than it should, but it removes the predictability, and this can be a good thing.
With either one you'll want the appropriate version of RSRDC, which makes the convoys historically accurate.
OM is indeed a full-on U-boat campaign, like SH3. I have some complaints, but it is the best thing going for the German side, and it is very good. It was made by Lurker, who also did RSRDC.
There is a GUI mod for SH4.
http://www.subsim.com/radioroom/showthread.php?t=152590
One thing you will probably want is a way to have more than one installation, so you don't have to switch mods from American to German. This is made easy with MultiSH4.
http://www.subsim.com/radioroom/down...do=file&id=433
For a full stop I just use the ` key beside the 1. I don't stop underwater, but that's the easiest way to do it.
Environment: I use OMEGU, the companion to OM. There are several good environment mods for the Pacific. I believe Ducimus incorporated one into TMO.
According to Lurker's post here
http://www.subsim.com/radioroom/showthread.php?t=134922 OM does add the schnorkel. I've never played far enough into the war to find out, but there it is.
Anyway, that should get you started.