Depends what you're facing.
On the hole the pulse laser would probably be my choice as most tanks I've seen go up to armour 12 (a Predator is possibly 13 on the front, but that's when the Falcon just uses its speed to get to the side armour). Also, giving my ability at rolling dice then the more dice I have the more likely I am to actually do something! That's part of the reason I like Orks
On the other hand, if I was facing a Land Raider then I think the Bright Lance may be more useful, even if I do only have a single dice roll. I was going to mention the Monolith as well, but doesn't it have the "Living Metal" rule that stops things like the Bright Lance decreasing its armour?