I personally wouldn't have thought that fleet types would be the big issue, whichever airline/ parent company were to buy it would gradually change to the fleet of their choice. First Choice already have an Airbus Fleet if that makes any difference, whereas TUI are all Boeing.
I would have thought that it would be the market share that any purchaser would want to get their hands on, which again, as First Choice is actively trying to move out of the lower end of the IT market (a shrinking market due to the lo-co competition) then a purchase of My Travel would not seem to fit with this strategy.
PP