The prospect of 4+ Flyrants would make me veer towards the Hydras, but the Manticore runs a close second. I just don't see Leman Russ Battle Tanks being all that effective a lot of the time, especially compared to the specialist versions such as the Paskisher, Executioner or Eradicator (an underrated and underused tank!).
Here was my list for the first game:
Platoon Command Squad in Chimera with Lord Commissar
2 Troop Squads, 2 Grendade Launchers
Heavy Weapon Squad 3 Lascannons
Special Weapon Team 3 Flamers in Vendetta for linebreaker/late objective securer
2 Vet Squads in Chimeras, 2 Plasma Guns
Leman Russ Punisher
Squad of 2 Leman Russ Battlecannon
5 Scions, 2 with meltas in Vendetta or deepstrike
Aegis Line with Quadgun
I got lucky and intercepted a Heldrake as soon as it came on the board and scored two weapon destroyed on two pens, saving a lot of my troops from the flamer. The 2nd heldrake was destroyed on the last turn, giving me victory due to a secondary objective called "The Last Laugh" (last person to kill a unit) and I won 15-13 according to the 2015 NOVA Open 40K GT rules.
I only get one more game to play before I have to solidify my list, and will face the Chaos/Daemon army again, then 2 games each against a specialized Space Wolves army, and a Nid Flyers army. We are in a 4 person bracket because we are isolated from a large player group.