I play both and I enjoy both. They are different, so don't expect a USN version of SHIII. Radar that really works is one big difference. Also, especially early in SHIV (1942) there is a lot of potential for shallow water work (eg. S. China Sea, Dutch East Indies, Sulu Sea).
If you try it out (and you should), my favorite way to start the war is in a Sargo-class boat out of Manila
