There's no reliable way to say a 5.1 would be better. And how do you define 'better' ?
The 5.1 setup you'll typically find is made for movies and games, which require a particular sound (bright and thumpy).
2.1 and more accurately, 2.0 is more music oriented.
That said, you can find 2.1 setups where a single speaker costs way beyond the cost of the best bundled 5.1s (like Logitech),
and performs way better in terms of music reproduction.
Or you can create your own 5.1 as well. (B&W, Dynaudio etc)