AVG or Avast for antivirus?

I just scanned my Flash drive with AVG (updated; free edition) and it found "No Threats Detected" But when I used Avast! to scan my Flash drive, it found the "(virus-name).scr" and when I clicked the "Delete", my MS Word files has gone, except for Excel and Powerpoint. What do you think is the best between AVG and Avast? Thanks!