Brand preference is subjective. Just for the sake of argument I'll talk about the Pioneer unit in my Town Car.
Also I don't know the extent of your knowledge so I'll just be thorough.
DIN and 2DIN are industry standard sizes. Depths will always vary.
You can get a 2DIN receiver or a tablet-style with a DIN or 2DIN chassis. Tablet style units generally have a screen with multiple mounting options but likely the width alone will impede 2 of your A/C vents.
Next, choose if you want the unit to play media (CD/DVD), have Android or Apple compatibility, or have expandability so you can add Navigation, satellite radio, or external media sources like your iPod. If you choose to play DVD's you want a 'parking brake bypass module' for your unit. Unfortunately iPod media seems to be only a fraction as popular as 10 years ago. You might want to see a few units with an iPod hooked up to it. I HATE the way my Pioneer interfaces it and never use it. The 20 year old Eclipse AVN unit in the Marauder makes iPod access fast and simple though.
Practically every radio will have four channels of speaker level output. You'll want one with a dedicated subwoofer preout (the RCA plugs), more of that later. Also onboard crossover settings (more on that later). Also, rear camera in, USB in, Bluetooth audio and hands-free calls. Also, if you plan on utilizing your steering wheel controls, get a unit that supports that feature. It will require an inexpensive adapter.
As far as my Pioneer unit, it has the ability to change the home screen to any photo you wish as long as you alter the size dimensions. My unit has unlimited illumination choices chosen on a colorwheel to match any OE buttons colors.
Let's say you've found what your looking for and you're ready to install it OR have it installed.
On the stock wiring there are TWO light blue/red wires. One is the steering wheel control wire, you cannot test it. The other is the radio illumination trigger. You CAN test this but it only dims the radio if you cover the sunload sensor on top of the dash.
If you choose Navigation or something with vehicle parameters, the speed sense wire is on the radio harness. I believe it's light gray/black.
Back to onboard crossover settings and subwoofer control.
If you use radio power for the four speakers you'll turn the crossover point on to 100Hz or so. That allows the speakers to play clearly without trying to reproduce bass frequencies they're physically incapable of playing.
If you still have the OE woofer on the trunk you can play that with the sub level control from the new radio and set that crossover to below 100. Try 80 or 100 and see what you like. Also, the woofer has a small dedicated amplifier that requires a turn-on signal from the new radio. It WILL have a turn on/off pop unless you get the Ford inline resistor. All this is on the Crutchfield site and it simple to find. The new radio has more power and more signal so the woofer will actually sound pretty good. You'll also be able to adjust the bass level with the new radio.
Like I said - almost all of these are similarly featured and operated. It's a matter of brand loyalty, quality, or budget.
