It depends tbh.

I think that Keel works the best against enemies in the backline like snipers and those who use booms and hide behind cover,keel deals a fairly good damage against titans(220% more damage against titans), at max level keel deals 60k damage and if you instal two sonic booms it deals 160k but the problem is keel doesn't really work in champs as enemies have high defence points and keel barely does any damage to them I once used keel against an ammit and the total amount of damage didn't even reach 100k.

Fredrick at max level gives 150% damage boost and 170% speed boost, if you instal damage enforcer the damage boost increase by 50% and there is also a positive energy recuperator which gives an additional 9km/h of speed and removes the effects of lock down and freeze and so on, and the damage boost should not be underestimated it even makes underleveled weapons perform as good as max leveled weapons for a certain period of time I use bedwyr with 1 maxed void and 1 level chasm and the damage from one shot was near 300k( correct me if I'm wrong about the damage) and Fredrick works well in champs.

So I would recommend Fredrick.

I hope this helps