Turion is a laptop processor similar to the Intel Centrino, a Sempron is similar to the Intel Celeron and versions can be found in both lappys and deskys. A Turion at 2.0ghz is going to be significantly faster than a sempron.
not only that but it is also more efficient. i believe the mobile sempron is 32 bit versus the turion64 64 bit processor. the turion runs cooler and consumes less power