I believe the IOTM should be the default for two reasons.
First and foremost, familiarity. Admittedly, the argument "We should look like Windows" is not very appealing, but I think the value of familiarity for *new users* cannot even be over-estimated (it seems to me that most "How to switch to Linux guides" have agreed that familiar apps and desktop paradigms are helpful). And as IOTM is used by Windows since 2009 (Win 7) this should be our default as well instead of the olden TTM which equals more the old Win XP panel.
Second, usability. One thing TTM cannot do - and probably never will due to the unpredictable position of apps in the panel - is launching and switching apps with Meta+[NUM]. At least to me this feature is huge for when using the keyboard it is the closest I get to the one-click-starting-and-switching-apps for mouse users. Plus, less horizontal/vertical space usage (preventing duplicated icons, wide windows due to text, multiple windows per app).