We have even more information now regarding the new Taurox and Scions for the Militarum Tempestus. It seems that a Taurox Battle Cannon is not quite the same Battle Cannon of a Leman Russ, but the Taurox comes with some great weapon upgrades regardless.
Please remember that these are rumors as we wait for an official set of rules. If you missed the first set of rules on the Militarum Tempestus please just follow the link.
http://natfka.blogspot.com/2014/03/militarum-tempestus-taurox-and-scion.html
via an anonymous source on Faeit 212
The battle cannon on the Taurox Prime is a 'Taurox Battle Cannon'. I don't want to rain on anyone's parade but it is just Str 7 Ap4 Blast. It can be upgraded for the price of a sergeant to the Gatling Cannon which is Heavy 10 Str 4 AP - or a further sergeant to get the Missile Launcher which is your standard Heavy 2 Frag/Krak combo. The Prime also comes with a mounted Hotshot Volley Gun like the Scions can take which is TL Salvo 2/4 Str 4 Ap 3. Both the Taurox and Taurox prime have 2 additional firing points.
Scions also come with Deep Strike and Move Through Cover. 5 of them come in at the same cost as a 5 man marine tac squad, additional models are 1 Eldar Ranger each. The Scion command squad is the same cost as a 5 man Assault squad, can be upgraded with a Medic, Vox Caster and Banner. The Tempestor is Ldr 9 and all Tempestus squads within 18 inches can use his leadership for morale, pinning, etc. In addition, the Tempestor is considered a junior officer and can give 1 command as per current IG rules. Any command squad models not upgraded may replace their Hotshot Lasguns with Hotshot Volley Guns, Meltaguns or Plasma Guns.
And the Taurox Prime can be taken by regular Scions as well, I had made an error in the previous bit.