I play both games, and won't uninstall SH3 for sure.
I love SHIV and plays it vanilla, for the moment. Just a couple of mods, to fix a small merchant CTD and to add an angle measuring tool centered on my sub on the navigation map.
But I'm aslo afraind of SHIV, with those few handy tools we didn't have on U-boats: the position keeper, the stadimeter distance measurement made easier, the default speed measurement that works great and even give you the ship course...
I'm afraidn of getting too much used to an "easier life"
So I play both, because SH3 with GWX is still a really great game too, and it will remains for long on my computer.