Answer
$F_E$ dominates.
Work Step by Step
Let $q$ be the charge on the particle.
We can write an equation for the magnitude of the force due to the electric field:
$F_E = q~E$
We can write an equation for the magnitude of the force due to the magnetic field:
$F_B = q~v~B$
When $v = \frac{E}{B},$ then the magnitudes of these two forces are equal.
If $v \lt \frac{E}{B}$, then the magnitude of $F_B$ decreases while $F_E$ stays the same.
Therefore, $F_E$ dominates.
