Personally I think both leader choices for 1500 points is overkill. I would stick with Belial and spend the leftover points on upping both bike squads to 6 bikers.
Also the heavy flamer is kinda "bleh". Get a cyclone launcher on the shoulders of a lightning claw terminator. Your gonna destroy anything you assault anyways, you dont need the heavy flamer for that. The cyclone gives you some anti vehicle shooting on the deepstrike. I would throw in a TH/SS terminator in that squad though.
Ye, I considered that. Don't know why I thought the heavy flamer was going to do anything. But considering there are tonnes of ork players in the tournament, it might be a good addition.
I will go into the idea of having Sammael in my list a bit.
The idea of having Sammael there on the land speeder is the fact that it is such a major support choice. It can pop up anywhere and add supportive fire, where the bikes are more or less a delivery system of termies and tank hunterss/late game objective stragglers. Sammael also makes the bike squads scoring.
The idea of the land speeder, is the fact that it's a land raider that can grab cover saves easily. He can zip around late game and contest objectives, and also draw some fire that would otherwise pick off the terminators like high-ap high-strength stuff, which I really don't like.
He also gives me a second apothecary, which is REALLY good. I honestly can't stress the importance of apothecaries in a deathwing/doublewing list. With precious few models, any failed save is disastrous.