Well I just recently gotten into SH4 and 5.

I started on SH5 first, then after seeing IGN's high rating for SH4 decided to give that a go. ( I tend to start with the latest game hoping to get the most out of my vid card and then work my way back ).
Anyway, I like the ease of making way points in SH5, left click set / move ~ right click delete, nice and easy.

If SH4 had that easy way point interface I would probably play that over 90% of the time. I found SH5's simpler interface great for getting the feel for the series and learning, I also liked the polish of the navigation system. However it's the same thing that I like about the game in terms of a simpler interface that made the game a little tiresome and dare I say boring shortly after playing ~ the challenge wore thin.
This is when I decided to try SH4. Now that game makes you work ~ but work that quietly exhilarates with satisfaction when challenges are overcome.
Both are good depending on what mood I'm in though. If I want a little more relaxing subbing then I go SH5, if I really want to get into a deep game I go SH4.
Now I'm just starting to get curious about trying SH3, another high IGN rating

~ but I'm saving that for atleast until after I get torpedoing down in SH4.

Anyway ~ Permission to come aboard, Captain