I played the SE series since version II and I played also Galciv I to some extent. I think both game series are good games and I like them both. Its difficult to compare them but I will try it a little bit.
Pros of Space Empire 2-4
-great micromanagement of vast fleets of ships and hundreds of planets
-totally moddable (at least version 4)
-deep technology tree
-interesting tactical combat system
-Spacecraft Carriers
Contras of Space Empires:
-sometimes unnecessary complexity and an rather incompentent AI (with the TDM modpack its better)
-inferior diplomacy (too many and unnecessary options)
-stock game techs often unbalanced
Pros of Galciv I
-easy to get in and to understand
-great graphics (at least for this genre)
-great AI
-excellent streamlining
Contras of Galciv I
-Micromanagement is sometimes too "light"
-only singleplayer
-no tactical combat
-no Spacecraft Carriers
And now the most important, which nobody in the above post cared to mention:
Space Empires V tactical combat will be REAL TIME! (boo, boo) As a long time fan of the series I am not even sure I will buy it, because of this new approach (maybe the game designer wanted to be more "modern")