Might be best to start with SH3 and the GWX mod. It's the most established and stable. If you start with SH5 and are disappointed with it, it won't seem like such a huge step backwards in graphics to go to SH3 at that point.
Also depends on your area of interest. SH3 & 5 are German U-boats in the Atlantic. SH4 is USN submarines in the Pacific.
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