I personally do not like either one.
BitDefender's statement of "
BitDefender Antivirus Free Edition uses an intelligent mechanism that can automatically take the optimum decisions to provide ironclad security without slowing down your computer." is rather an understatement. On my time using it, it did slow down the system and it was a mid range system.
Lavasoft's free solution's statements of "Fast Antivirus" is also an understatement, at least since I last used it back in November.
They also have questionable RT and Web Browser protection.
If you take the performance question away and focus on protection/modules, then BitDefender is ahead, but I would still not recommend either one.