Nobody really knows since this information has not yet been made public.
I would personally assume, however, that the TB will receive Gingerbread first before any upgrade to 2.4. The TB has had prototypes available since last August and HTC is usually the faster than the other manufacturers at skinning Android with their custom skin (in this case, Sense) so I would think that they have been working on a Gingerbread variant since the day Gingerbread became available. Android 2.4 was just barely announced and is not yet available to the manufacturers. HTC would not sit around and wait for months, I believe they're working on GB for the TB as we speak and have been for some time
The TB is "older" than the Incredible S in this regard and, therefore, will most likely play catch up to 2.4 instead of jumping right to it