R:TW is a brilliant game, rushed and bugged here and there, but a great game.
The RTR mod was just undescribably excellent.
Now, Barbarian Invasion has a few good points, like new formations, and this and that. But really, I have yet to find many differences between this one and R:TR. Some bad points here and there as well, like the fact that eventhough you're one half of the Roman Empire, you don't have a map of the other half.
Don't get it, unless you can get it cheap or free.