MarkAble is better in my opinion because it has a audiowizard option. You can put in multiple cd disks from a audio book and then joins all files without the 2 second gaps between files. This makes it one fluid and continous seemless file. If you use it with a Ipod mp3 player it is still bookmarkable. It also of course has a manual mode where you can join any # of tracks together and still eliminates the silent gaps between mp3 files.
Not 100 % sure because have not used
Audacity but I believe with Audacity you have to manually remove the silent gaps between mp3 files to make the book seemless. and no bookmarking or skip when in shuffle mode on Ipod . If you have a book file using audacity and are in shuffle mode on a ipod mp3 player with songs the audio book can come up while listening to music. That certainly would be annoying.