It seems most of the super mods do alot of the same. I was new in your shoes just 2 months ago. You stumbled across the right place

.
But the mods do boil down to major differences in the end, after all the general fixes and such (some have enviroment packs and the like) but at the end of the line after they all do similar things the differnces are:
TMO seems to make the game harder, some reworked layers in the campaign.
RFB tried to emulate as real as possible, I tried it even with map contacts turned on your map doenst show anything, I cant remember if it gave sonar contacts on the map either ( some one can prob answer that)
OM Takes SH4 back to the atlantic and puts you back in U-boats so if youd rather play a uboat than an american fleet boat thats what you want.
And RSRD, reworks the campagin layers in the pacific to include most if not all the major battles, and i think it makes every boat out there follow close to what happend, they start in ports and follow real shipping lanes to their destination, they dont spawn randomly in the middle of nowhere or you wont find Lone jap merchants in American waters.
That being said I personally play on my lap top TMO currently with RSRD, but im thinking of taking RSRD off, I love WW2 history but Id rather play the game with "my own war" rather than alt tabbing to wikipedia to find major battles and such. Id rather have some more frequent contacts so I can learn the ropes, then maybe put it back on so I can relive the historical war.
On my desktop I run SH4 with OP Mon and OMEGU (thats the enviroment mod that makes the game look beautiful as well as changing some things im not too sure about.)
whatever you pick you cant go wrong. All the super mods are masterpieces in their own rights. Read the threads in the mod sections to get more indepth details about each one, and theres a program called JGSME that allows you to take on and off mods very easily. Information here is a plentiful.
Welcome aboard from another newer sub skipper

.
Hope I helped and if I failed anywhere in a description I apologize and im sure some one will correct me
EDIT: forgot to mention each supermod is standalone except RSRD. I belive that can be run with each of the supermods but check out all the mod threads for install instructions.