I find it depends, I feel like the thermal takes my shooty knight too close. However, now that it is d6 instead of d3 shots it is tasty for shooting tanks. The rapid fire battle cannon I feel is just more consistent for taking on multiple targets without being super focused. Really good for dealing with light tanks and TEQ. I normally run one of each in my Knight list to spread the benefits around. For support to my space wolves I would go battle cannon as I have other anti-tank options
Actually the main reason I am considering the change is point savings.
I could add 3 hunter killer missiles to my leviathan. He has bs2 reroll 1s if i spend 2CP the turn he arrives from outflank. So i trade D6 BS3 RFBC shots for 3 one shot rerollable BS2 HK missiles wih much better odds of hitting.
Gives me more overall anti vehicle abilities than i currently have.
I can also upgrade my WGTDA pack leader to a chainfist
Only problem is i dont have the actual bits. need to chop and magnetize barrel i think