All four of these manufacturers are quality, well-engineered designs, and you really cannot go wrong with any of them as an iPhone or an Android user. Why choose one over the other? How are they different? Let's first talk about why you don't want just any Qi pad on the market.
The 7.5W Apple wireless charging standard
When Apple released the iPhone X and iPhone 8, the company deviated from the Wireless Power Consortium Qi 1.2.X standard, which can charge at the original 5W, a higher-level 9W, or the newest 10W, 12W and 15W used by Samsung.
Apple chose 7.5W for its Qi implementation rather than go with 9W, 10W, or 15W. There is no apparent reason why it decided to do this, and to date, it hasn't said anything about it, but if its cancellation of the AirPower charging mat in 2019 is of any indication, my guess is it may have something to do with heat.
That means, while many inexpensive Qi chargers you can buy on the internet advertise "fast charge," if you use them with your new iPhone, the best they are going to do is the lowest common denominator, which is 5W.
We found this out the hard way with a large number of low-priced Chinese charging pucks. They cannot charge current generation iPhones at native 7.5W speeds; they can only do it at 5W, so buyer beware.
You've probably noticed that your Airpods and other Bluetooth headphones (such as Anker's SoundCore Liberty Pro 2, Google's upcoming Pixel Buds, and Microsoft's Surface Earbuds) may have wireless charging cases, and your Apple Watch also has a magnetic charging connector. Can you use just any charging pad to charge them? The answer is, maybe. Possibly.
While the AirPods charging case uses Qi, there's no guarantee that a third-party charger is going to mate correctly with it unless it is specially designed for it.
Additionally, the Apple Watch itself doesn't use Qi; it uses its special magnetic connector. That's why several manufacturers, such as Belkin and Mophie, have made 2-in-1 and 3-in-1 charging docks to support the iPhone, the AirPods, and the Apple Watch, simultaneously. Apple attempted to create a 3-in-1 pad to support all three products, the AirPower, but it failed to launch the product due to difficult engineering issues.
Google's Pixel 3 was unable to achieve wireless charging above 5W unless you used the Google Pixel Stand. For Pixel 4 and Pixel 4 XL, things have improved somewhat, but you'll still need to be a bit pickier and choose a wireless charger that supports the Extended Power Profile standard -- also commonly referred to as EPP. Most of the new wireless chargers should be outfitted with EPP, but older models likely don't support it. You'll want to look at the specifications of the product to be sure that it does. Google's Pixel Stand absolutely will charge the phone at native speeds, as will the products noted above.
You can buy any number of charging sleeves or receivers to retrofit an older iPhone or an Android that does not have built-in wireless charging. I've seen them as cheap as $5 and up to $15, such as the Cloele, which I had some good success with using my wife's old iPhone 7+ and an OtterBox Defender case and inserting it between the rubber backing and the plastic.
However, be aware the best you are going to be able to do is around 800mAh to 1,000mAh. Because your phone has not been designed for this, you may experience heat issues -- mainly because the distance between the coils and the charger base will not be the same as a phone that has this built-in, so alignment will not be ideal.
Every case has different thickness and heat exchange characteristics, and not all receiver coils behave the same, so your mileage is going to vary. How have your Qi charging adventures gone with your new iPhone or your Samsung device? Talk Back and Let Me Know.