Silent Hunter 4 U-Boat Missions. Wide variety of supermods and smaller tweaks makes it a much more enjoyable modding environment. Lots of "competing" modders without strife. It's a community thing.
And there's much left to do on SH4. So there's more room for improvement there. SH3 is in a much different stage of life. It's had a great run. It rehabilitated the submarine simulation genre. It established the Romanian developers' team as the one to bring the franchise forward. Now it's time to move on with a grateful heart.
SH4 has a good legs still and can troop on awhile longer.