[Exakt]

Here is my opinion:

SH3 is better than the current state that SH5 is;

Good points
SH3:
-Full war, 1939 to 1945
-A whole selection of boats.
-Far more mods right now available (such GWX3, which is a must have).
-Stock, SH3 feels more real than SH5
-"True" dynamic campaign.
-Even with a 5 yrs old engine, graphics are still on part of some newer titles (except for the stock resolution)
-Can work on almost any PC (I recently had to revert to an ATI X1650Pro and it can run this game with GWX with hardly a stutter).
-Digital copies without StarForce or other dreadful DRM.
-Good tutorials.

SH5:
-Nice graphic
-Able to walk the entire boat.

Now the bad:
SH3:
-Stock has bugs (most of them corrected with mods)
-If you have an original DVD, (just like me) you have to deal with StarForce, which can be circumvated.
-Graphics are starting to show their age.
-No Wolfpacks.

SH5:
-Interface is cumbersome and non-intuitive.
-Have to practically walk to each station to do something.
-Awful implementation of RPG-style gameplay
-A resource hog even on my previous video card.
-Awful DRM.
-Lots of bugs (such as if you encounter a friendly sub, their torpedoes will just get out of the sub and then sink to the bottom).
-Still no Wolfpacks.
-Campaign ends before the end of the war (watch for an expansion I'm sure).
-Useless tutorial.
-Limited to a boat (the VII series).
-AI is clueless.

Personally, I gave up on SH5 for all the bad points I have stated above.
SH3 is still keeping me hooked.

And just to tell you my story with SH3, I bought it about 4 years ago, installed it, played for about 20 mins and then shelved it for about 3.5 years. Reinstalled earlier this year (or late last year, can't really remember the exaKt date) and now I am deeply "committed" to it. The last few games that got me hook like this were the Pre-Vegas Rainbow Six, Ghost Recon (the first one plus its expansions, not the GRAW stuff), Dawn of War and its expansions and FreeSpace 2 (which the SDK has been released about a decade ago and now it could compete with a lot of space sim games of today).