Depends on what you mean – first to third title or the youngest three times world champion? If the youngest, it’s easily Vettel, as he’s 6 years younger. The record for youngest ever three time world champion is 31 (I think), by Senna and Schumacher, not sure about months/days. Alonso will be 31 next year as well.
It’s very likely Vettel will win another title in 5 years’ time, and he’ll probably be the youngest ever triple world champion.
As for who’ll be there first, hard to say. The two are, in my opinion, the best two in formula one at the moment and if Ferrari can produce a good car from the start it’ll be tight.
Honestly can’t say and that’s why I’m very much looking forward to the battle.