Spectral Fields are most useful against blitz attacks as they often use short-ranged Energy weapons such as Burst Rays and Thermal Beams.
They also carry the most energy, maximizing protection against Alien Damage and collateral damage, as well as attacks that involve a mix of weapon types. They are the standard shield to be used on most modules.
Spectral Shields are very heavy to equip due to their massive shield capacity. This can limit the armor or weapons available to your modules.
Modules with active Spectral Fields are encased in a blue dome, which occasionally pulses yellow but is hardly noticeable.