HI @TussalEmpress57.
There are two things for you to consider. Firstly, what store do you want to buy from:
Windows Store
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Play on your computer, or on Xbox with Play Anywhere
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In-marketplace purchases are charged against the credit card on your Microsoft account, meaning you can find your purchase history on your Microsoft Account Page
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Does not require you to install a marketplace on your PC (assuming you have not removed the Windows Store App)
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The Standard version is available on Game Pass for a small monthly fee. This subscription unlocks several other games including Halo MCC, Age of Empires, and many more
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If you get the game through game-pass, you will be able to buy it outright later through the Windows Store at a discount.
Steam Store
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Does not include Play Anywhere (If you want this you need to buy from the Windows Store)
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In-marketplace purchases are charged against your Steam account. This can cause issues if you have a purchase problem as you may need to get support from Steam, or from Microsoft
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The client download seems to be a bit more stable
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Not as many game files are locked
Once you have chosen your platform, your next decision is what version you want.
Fundamentally, the only difference is the number of aircraft and bespoke airports the sim comes with. Each tier has five more of each than the other. It is possible in the future, that the higher tiers will get more freebies, but nothing has been announced to indicate that yet.
If you are wondering what extras each version comes with, the store page should include a full list. The one warning I would give you is to buy a premium version now if you are leaning that way. You get a significant discount if you buy a bundle of the Microsoft/Steam store. If you choose to upgrade later on, through the in-game marketplace you will pay quite a bit more.