Firstly, it might depend how old your son is or what he's interested in. If he likes trains and the old west, then it's a no-brainer for Colt Express. If he likes jungles and temples, then it's Cacao. If you want something similar to the familiarity of the tile-laying nature of Carcassonne, then it's obviously Cacao. If you want a different type of mechanic, then go for Colt Express.
I haven't played Colt Express yet (though I will purchase it soon), so I can't compare the two. I suspect Colt Express is more pure 'fun' because of the 3D train, the moving of the characters between carriages, etc.
Here are some random thoughts on Cacao:
- I like it. The art is beautiful, the theme is fun, it's quick, it's simple but still with 'hidden' depth.
- The main reason Paul started the tile-laying thread was that those of us who love Carcassonne are probably naturally drawn to tile-laying games. And that's true with Cacao; the laying of the tiles and the growing of the board is equally as pleasurable with Cacao as with Carcassonne. For example, I introduced Cacao to my regular Carcassonne partner last weekend and she loved it. We then played it again a couple of days later.
- A few differences in the tile-laying between the two: In Cacao you don't have the 'excitement' of drawing single tiles, so you can't really have that moment as in Carcassonne when you pull exactly the tile you need to close a big city. On the other hand, you could say that the placement rules of Cacao mean that the board develops in a more consistent way. You don't have holes in the board or a line of tiles going off to the side (as with some Carcassonne roads), so at the end of the game you have virtually a perfect square or rectangle of tiles and the vibrant artwork makes it look really colourful and great.
- Both Colt Express and Cacao have scope for expansions and are starting to produce them, so that greatly adds to replayability for both. One thing to note about the Cacao expansions though is that because of the sun disk rule, you can't really play mega-Cacao in the way that the expansions and rules are currently constituted. That is, mini-expansions and the Chocolatl expansion that's coming soon add jungle tiles but not worker tiles. Because the sun disk rule requires the proportion of the two types of tiles to remain the same, you need to remove jungle tiles from the game if you add new expansion jungle tiles. I wonder if they will start to 'correct' this in future expansions by adding more worker tiles (e.g. a 4-0-0-0 orientation). Or you could buy two copies of the base game and use these worker tiles with all the jungle tiles. Personally I don't mind this because I'm not a 'mega' player, but what it means is that games with expansions aren't any longer than regular games, unlike Carcassonne.