You forgot image compression. From Google...
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You can automatically save your photos and videos to your Google Account when you turn on backup. Learn how to back up your photos and videos. Each Google Account comes with 15 GB of storage and you
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All images and videos uploaded using the "High Quality" (unlimited) preset are being further compressed. Keep in mind the "original" jpg or HEIC are already compressed formats derived from the original capture on the device.
Here are two images I pulled out of Photos (IMG_0677.jpg and IMG_0687S.heic) then uploaded and downloaded with Google Photos.
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IMG_0677.jpeg is a fairly basic image captured with an iPhone 11 Pro Max. The original capture and stored images on iCloud is 2.9 MB. Uploading to Google Photos compresses this to 1.3 MB.
IMG_0687S.heic is still a fairly basic image captured with an iPhone 11 Pro Max however the image is a bit more sophisticated. The original capture and stored image on iCloud is 3.3 MB plus it has a resolution of 5648x4240 (image capture outside the frame used for editing and the iPhones feature set). Uploading to Google Photos compresses this to 1.6 MB and 4616x3465 (=15994440 aka 16mp). While the file extension displayed as HEIC on the Google Photos....Downloading IMG_00687S.heic will result in Google Photos changing the file extension on export. This is a screen grab from Google Photos image info for IMG_0687S.jpg prior to downloading (might be a way to avoid this but its a moot point because the images are bit for bit)...
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There is a silver lining though, Googles image compression algorithms are amazing. Uploading an image that is less than 16 mp is virtually indistinguishable from the original uploaded file! IMO its truly amazing.
So smaller image file size using magic compression that is indistinguishable from the original is better right? Not exactly, actually there are some glaring issues that can even effect casual users that strive for quality.
RAW images even under 16 MP (even taken with the iPhone) will be compressed and colors, luminance, etc will be obviously changed. Even images using other files types with less compression can be negatively effected.
The main issue is the process for editing. Every time you download, edit and upload you are adding another layer of lossy compression further losing image data. This is IMG_0687S.heic from earlier downloaded from Google Photos as IMG_0687S.jpg, edited by typing the 4 letters of text on the image (IMG_0687S-1.jpg), uploaded than downloaded again. That sounds tedious however basically that is just ONE edit.
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The image went from 1.7 to 1.2. At this point the lossy compression can be seen with side by side comparisons. Further edits will have obvious degradation of image quality that won't require side by side comparisons. This is very similar to comparing images sent iMessage and standard SMS/MMS. If you keep sending the COPY of an image iMessage will not degrade image quality, indefinitely because its sending the original. If you do the same with MMS the image quality of the COPY degrades exponentially every time its sent.
Now that isn't the end of the world, you just need to keep this in mind if you edit your photos. If you edit a photo keep the original so if you need to edit it again you can work from the original.
You get what you pay for IMO. If you do not want to compromise image quality you'll eventually need to pay for their 'Original Quality'. For me personally a native app shared across all devices that work from original images that contain metadata specifically for editing and reverting from edits in one file is easily worth the extra 10gb Google offers.
Not knocking anyones decision. Do whatever works for you and there is no doubt Google Photos is a good service.
EDIT: We should keep in mind that iCloud is more of a mirroring service while Google Photos is more of a back up service that has the benefit of easy access. If you have 10 Apple devices they'll have the same photos, messages, notes, calendars, documents, desktops (Mac), music, etc etc etc by using the same iCloud account. If you want every device to have its own 5gb of iCloud you just need to use a different iCloud (same with Google in this respect).