In terms of being a really-well-tested and improved game, of course SH3+GWX is right now the way to go. But I personally believe that the real choice is more one of theaters than of "games". If you are a fan of Pacific submarine warfare, SH4 is for you....if you prefer the legend and glamour of U-Boots go for Sh3. If you just want to approach the subsims world, any of them will be good for you.
