MPLS was originally designed for ATM, to make it better and faster. Now it is used over Ethernet or other IP enabled networks, as a layer 3 tunnling protocol. It is more economical than ATM usually. It offers the same general QoS as ATM, often even better. It uses IP, which is generally not used over ATM, it is tunneled within ATM frames. But the frame sizes are not fixed like ATM, so speed is generally faster, but more jittery.
My company sells MPLS to existing ATM customers all the time, but never the other way around. Some of that may be due to the fact MPLS is faily new in comparison. AT&T has had it longest, and so far they are the only ones with MPLS edge to edge, but the others are catching up quickly. And everyone has some MPLS offing now.
It is the wave of the future it appears.
I hope that helps.