Chaos in the Old World really shines with the original 4 powers, but it is not bad with 3.
The game needs at least 3 to play.
The expansion adds a fifth player, and also some alternative, optional power cards, which change the way each of the 4 original gods have to work on their victory points.
Twilight Imperium 3 can be played with 3 and more, and scales better than Chaos, but the game is a lot better around 5, 6 players.
I have never played with 8 (with the first expansion you can play with that many), and people say it is almost insane to do so, since the game length increases a lot.
Chaos in the Old World is also a lot easier to learn than Twilight Imperium, that might be another good factor towards it.
Both games are very different, Chaos is a territory control game, but where you focus on doing things that make your god in particular to get points. Also it is a game where you are constantly screwing with each other, and I love that.
The game is very thematic, if you get to know the Warhammer Fantasy universe you can see how well done the mixing of mechanics with the theme, the designers did.
Twilight Imperium 3 oozes with theme also, and has more options and different races than Chaos, but the player interaction is something that will happen sometimes only midway through a game.
TI3 is a very deep game, the learning curve for the rules is a bit more steep, but worth it, if you like deep games.
If you want to have a look at a step by step tutorial, simulating a complete game, this website is great:
To be fair, even after reading the rules several times, one aborted game in the middle and a second complete, 3 player game.. after I went through that awsome tutorial, I learned I was doing some very important rules all wrong hehe.
I like both games a lot, but as I said, I like Chaos better. Not because it is a better game, but the combining of medium difficulty rules, medium length and great interaction among very different powers, make it a great game.
TI3 on the other hand, is a meaty, deep game, and if you have friends who like complex games with lots of different interactions, a game where you make long term plans... it is epic. That´s the word. TI3 is epic.
But TI3 being such a beast, is not the kind of game you will probably play several times. It will be a game where the game is the only event in the night.
Now, if you want to watch a very nice Chaos in the Old World video, go to youtube and search for "Chaos in the Old World - downtime town"
That was the video that "sold" me the game... I bought it almost instantly after watching it.
Recently Tom Vassel from Dice Tower made a 2, or 3 part video review of TI3, very good to see all the game has to offer.