I really like Europea Universalis. But to say it has more depth than Sins of a Solar Empire is not something I can remotely agree with. (1)
The fact that you admit that you don't see what culture does in Sins I think makes the case that you haven't fully explored Sins or more to the point, have decided that Sins doesn't have the kind of depth you choose to have. (2)
There are plenty of people out there that say Galactic Civilizations isn't a "real 4X" simply because it doesn't have the "depth" of Space Empires V. But that doesn't make what they say true objectively. (3)
(1) It might have more 'depth' but all that 'depth' is piled onto you (if EU is anything like Hearts of Iron). A newbie faced with such a game cannot grok even the simple things because of the complexity. The user interface is something integral to Sins. The fact that it's an RTS gives it a familiarity that doesn't come through with Turn-Based games, which all seem like a totally different monster by comparison. In Sins, you're playing the game, you're playing the ships, you're playing the movement, you're playing the moment. In TBS games you're playing the interface - and this differs WILDLY between different games - even with IDENTICAL mechanics.
The point is, just because Sins has a more logical UI and more logical gameplay doesn't make it less 'deep', just easier to play.
(2) This is where the real 'depth' of Sins lies. The first time I played this game, I thought it was practically BARREN compared to other games. But that's just an illusion. The illusion of the peaceful music and stillness of space. Other RTSs are so BUSY compared to Sins, and I think someone on the dev team realized this, because the music is ironically peaceful.
This game has levels of complexity. These levels are approached naively, nothing is ever piled onto the player. A lot of people complain that the 'ships for each race are identical.' How does one argue with that? Yes, I suppose they are, but the races are so DIFFERENT that the ship question misses the point entirely. Someone making this claim has not yet breached the outer layer of the game.
(3) The 'depth' of a game like Space Empires V comes in the form of extreme front-loaded complexity. After that complexity is overcome and you have memorized the encyclopedia of information that it takes to understand relational mechanics in the game, the depth is all gone.
Galciv II at least tries to break away from all that front-loaded complexity starting you out with very logical naively reachable goals and leading you through by letting you discover the game instead of fight against it. There is still a lot of complexity there, but the interface (remember, in a TBS you are playing the interface) makes it all come together without it all coming together at one time, overwhelming a new player. There still isn't a real 'depth' to the game since I've always felt that if the game was multiplayer, there would be only one or two real paths to victory. (It's still fun, though. And I consider whoever designed the interface for that game a god among ants in the gaming industry.) In this way, Sins has much more depth, even in an unbalanced state. The key is that it isn't as complex, it's not as front-loaded with mystery and voodoo.
Some people misinterpret this simplicity as a lack of depth. These people have apparently never played Go.