Plates and springs get lumped together as "the vintage reverbs", and both are mechanical: a signal goes in, a piece of metal moves, and a pickup turns the movement back into sound. But they move in opposite ways, and that opposite is the whole difference between them. Once you know it, choosing between them stops being a matter of taste and starts being a matter of source.
Why a plate sounds like a plate
A plate reverb is a sheet of steel under tension. When it is struck, the vibration travels through the steel as bending waves, and bending waves in steel are dispersive: their speed rises with frequency. Hit the plate and the high frequencies arrive at the pickup before the low frequencies do. Every reflection carries that head start, so a plate's tail has a faint upward chirp running through it, and a burst of dense, bright energy at the very front.
That is the texture people mean when they say a plate is "silky" or "shimmering". It is not brightness in the EQ sense. It is time: the top of every sound gets there first. A digital reverb that only copies a plate's frequency response and decay time leaves the dispersion out, which is why so many plate presets sound like a hall with the treble turned up.
Plate Daddy reproduces the dispersion directly and puts it on a control. Turn DISPERSION down to zero and the unit collapses into an ordinary reverb; turn it back up and the chirp returns. Doing that once on a snare is the quickest way to hear what a plate really contributes. The seven plate models (Big Steel, Steel SS, Gold Foil, Eco, Bronze, Pocket and Chrome) are different tensions and materials, and each has its own dispersion character: Gold Foil is the most restrained, Chrome has the most pronounced chirp.
Why a spring sounds like a spring
A spring reverb is a coil of wire with a transducer at each end. The signal shakes one end, the wave travels down the coil, and the other end picks it up. Below the coil's transition frequency those waves are dispersive too, but the other way round: in a spring the high frequencies travel faster than the lows down the coil in a way that stretches every echo into a short downward chirp. That is the drip. It is not distortion or a filter; it is each reflection arriving smeared in time, high end first, low end trailing.
Because the wave has to travel the length of a physical coil and bounce back, a spring's reflections are also sparse compared with a plate's. You hear them as individual events, especially early in the tail, and a knock on the tank sends the coil swinging in a way no plate does. That is the boing, and it is why a spring on a percussive source can sound like the source is being dropped down a stairwell.
Coil builds the drip rather than approximating it, with DRIP scaling it the same way Plate Daddy's DISPERSION does. TENSION retunes the whole pan, and BOING sets how much of the transducer thump comes through. Its seven tanks run from a drippy valve-driven surf pan to a dark studio twin-tank and a six-spring unit.
Which one to reach for
The plate suits sources that need to sit in a space without gaining a rhythm of their own. A plate's reflections are so dense that the tail reads as a wash, so vocals, snares, pianos and strings take a plate without the reverb competing with the performance. The upward chirp also adds air that a dark source can use. When a vocal needs to sound expensive, a plate with a long pre-delay is usually the answer.
The spring suits sources that can carry a rhythm. Guitars are the obvious case, because the amp-reverb sound is a spring by definition, but the same drip works on organs, electric pianos, claps and anything dry and percussive that wants to sound like it is in a room with a personality. A spring on a lead vocal is a style choice rather than a default; it can be wonderful on a sparse, retro arrangement and distracting on a dense one.
There is one more difference worth knowing. On a plate, the first thing you set is the plate model, because it sets the character more than any knob does; on a spring, the first thing you set is DRIP, because that controls how much the reverb chirps at all. Both plugins put Decay next, and both have a Freeze for holding a tail while you audition changes.
If you have never heard the two side by side on the same snare, that is the experiment to run. Both plugins have a 14-day free trial of the full version, so put Plate Daddy on one aux, Coil on another, and send the snare to each in turn.