Welcome aboard!
I've played SH3 w/GWX since April 2008 and have loved every moment. Well worth it. When I was looking for a subsim then, I wanted an American fleet boat game and saw SH4 but was turned off by the reviews about the bugginess and I wasn't really sure my computer would handle it. So I went with SH3 and have never regretted that decision.
Last month, I took the leap and purchased SH4 with the U-boat add on and have been trying the different super mods out there. I'm still learning the ropes of driving a fleet boat around the vast Pacific with TMO 1.7. I'm going to switch to RFB after my next patrol and give that a test drive for a few patrols to see what I like best. I'm just starting manual targeting in SH4 for my next patrol, so it might get more interesting(been doing manual targeting in SH3 for about 6 months now).
The jury is out still, but I lean more towards SH3. I like the game play better in SH3 and I actually am leaning towards liking the graphics better in SH3 as well with GWX.
I've yet to have a "Wow!" moment in SH4. I've had a few of those in SH3. I'm not giving up though. I'm still giving SH4 a fair chance, but by no means am I taking SH3 off my computer.
One more thing: do you want to play the underdog later in the war?
I like playing the underdog in SH3. U-boats start out being the terror of the Atlantic, but by 1943, you become the hunted and just surviving is a miracle. If you sink something(and survive), that's icing on the cake. I like that challenge, although I'm not that far into 1943 yet(June/July IIRC).
Another thing too:
Different theaters have a different feel in SH3.
The Med feels different than the Atlantic, and the Black Sea is a whole lotta different as well. I've got carreers running concurrently in all 3 theaters. Haven't tried the Indian Ocean.