Besides an "updated" User Interface, SH5 is ATO-based, and in most opinions needs extensive modding to make it playable. SH4, even the U-Boat side, is PTO-based, unless heavily modded, as in Operation Monsun from lurker_hlb3, or as in
Knights of the Sea Depths II, currently in development.
Now, if you ignore the U-Boats in SH4, the US Fleetboat side is an enjoyable gaming experience, even in stock trim with v1.4, but if you have the v1.5 Uboat Missions Add-on applied, it's even better...
