Two things separate the specifications of the P series from step-down Vizios with FALD, including the M-Series and the E-Series, as well as from the step-up P-Series Quantum (only available in a 65-inch size). Those things are its number of dimmable zones and light output. Zone number is an important spec because it controls how precise the dimming can be. More zones doesn’t necessarily mean better picture quality, but it usually helps.
Here’s how the 2018 Vizios compare to one another, as well as to TCL’s 6 series (Sony and Samsung don’t disclose the number of dimming zones on their FALD sets). As you can see, the TCL 6 series has more zones than the Vizio P-Series at 55 and 65 inches — and more than any Vizio aside from the Quantum — but the Vizios are available in larger sizes.