The Typhoon and Tornado are not upgrades. The land speeder squadron is for normal land speeders. These are the ones normally armed with a heavy bolter, upgradeable to a multi-melta (DA can also mount an assault cannon on them in addition to this). They are taken in squadrons of 1-3 vehicles. The Tornado and Typhoon are different land speeders. If you take one of these it is only a single vehicle (not a squadron of 2 or more), and occupies a fast attack slot. They cannot be taken in a squadron of normal land speeders, as they are a completely separate entry.