I'd like to convert MKV to MP4 by considering the following on Precise (64bit):
- resize (only want specify width and aspect ratio, height is variable between files)
- make use of quad-core CPU (do it multi-threaded)
- video re-encoding with MPEG4-AVC (H.264)
- transcode MP3 audio to MP4
- preserve subtitles (only one language)
So far I have no experience with avconv. If subtitles not possible with MP4, I can also stay with MKV container (and leave the MP3 audio untouched).