Why would someone want to use Tor through a TorVM in Qubes, compared to other approaches? This page compares TorVM to Whonix, Tails, and Tor Browser but doesn't come to a firm conclusion as to which is "best". This is because, as the article states, the various approaches have "Different threat model[s], different implementation[s], different use cases, although some overlap" and "Different political and design decisions". I am especially interested in the different threat models involved here.
Written in September 2013, this answer might be out of date after some time. Full disclosure: I am a maintainer of Whonix wrote most of this page, so I may be biased. (By the way, that page is under a Free as in speech license, so if you legally copy it to wikipedia (or similar) it can be made truly objective.)
If you are most concerned about local forensics:
Use Tails, because it is amnesic by design. (Or maybe Liberte Linux, but I know too little about it to compare it.)
If you are most concerned about vulnerability exploitation:
Use Qubes OS TorVM, because Qubes OS's (what Qubes OS TorVM is based on) developers seem to care most about strong security and isolation, which means, should one of your VM be compromised, the attacker might have a harder time compromising another VM. Qubes OS lacks Tor Browser and secure network time synchronization.
If you are most concerned about protocol level leaks and ISP snooping:
Use Whonix, because it includes the original Tor Browser (downloaded from The Tor Project's homepage), has unified desktop resolution for all Whonix users (who don't install guest additions), boot clock randomization and secure network time synchronization.