Stock GWX UBoats look much better than stock NYGM ones, largely because NYGM ones are unmodded in the visual part, (There are however reflection and periscope fixes, as well as the smoke exhaust mod included) while GWX ones come at least with a nice hi-res skin. BUT there is a pack of improved models done by Mykhail for NYGM which are higher-res and add several improvements, so you are probably referring to that.
To improve the GWX models you can add several mods currently out there, like floodholes by Rubini, exhaust mod, several improvements by flakmonkey (periscopes, shaders, etc) and of course the improved turms by Aces, which have a specific thread here:
http://www.subsim.com/radioroom/showthread.php?t=168825