Between those two, I'll go Kaspersky over Avast! due to the same reason as what
@Shran mentioned in his post above.
If you are open to another option/ suggestion:
1) Emsisoft IS - Considerably Light, install and forget, good BB.
2) CIS - Free, needs heavy tweaking but offer similar, equivalent protections level
3) Norton - Light, good detection, install and forget, SONAR.