WOW, some harsh criticism
I have beaten GalCivII @ masochistic and have beaten Civ4 @ monarch.
I personally find Civ4 to be the much harder game to learn, havning spent numerous hours reading posts trying to improve my play, whereas never having spent 1 minute in a forum for GalCivII. I play the Civ4 vanilla with no mods which is a much tougher game to play than BTS with bug mod, there is nothing to figure out with those mods, it gives you all the info you need instead of trying to figure out what is going on. Civ4 didnt give you any info anywhere so they made a mod to give people a chance.
With that being said, I find Civ4 to have more replayabilty just because it is that much more difficult to learn, I was able to learn GalCivII on my own but not even close with Civ4. I can get Bts and get the mods and win @ emperor, probably even immortal, but I personally think those mods are just fancy cheats.