if you're going for a low turn count, I'd say sticking with Vanessa. While Tana's growths are better and will come out stronger if both were promoted to 20/20 and compared, her starting stats are on the low end of the spectrum, which will cost you efficiency if you want to improve her. In efficiency runs, a Pegasus Knight is best suited for transfortation, not combat, so stats are pretty much irrelevant. As with Franz, he's also preferable or Kyle or his brother thanks to early game availability and thus more time to train him. His stats aren't half bad either, but if you're mostly using him to carry slower units, that's less of an issue. One should also consider Cormag, who joins the chapter after Tana in Ephraim's route or four chapters later in Eirika's route. He starts out a higher level than both of them, is another flying unit, and has more solid base stats than both of them, making him a capable fighter as well as a ferry. That being, my suggestion would be to stick with Vanessa until you recruit Cormag, and have him as your main from then on. In summary, Vanessa vs. Tana is the classic FE tradeoff of early game availability vs overall potential. For another example, we could compare Franz to Amelia, who joins in the same chapter as Tana. While Franz is playable from the first chapter, Amelia starts off at essentially level negative nine, giving her more potential stat gains but she's nearly impossible to train without skirmishes and the Tower, which in LTC will likely be avoided.