Official “in game marketplace.”
No proof of purchase. (In case of a dispute this puts you at an immediate and unacceptable disadvantage)
Really slow to update. (Sometimes weeks later than external vendors)
Content is encrypted and DRM protected. - this means that certain external apps cannot access deeper functionality (Navigraph can’t interact with airports for example)
The DRM protection validates on sim launch. Sometimes it doesn’t validate because of network glitches and your purchase is “unavailable”
No real rating/review system.
(There is a lot of junk being sold that’s not being properly rated/reviewed.)
SimMarket
Lots of stuff.
(Which also means that you will find a lot of junk there too)
At least there is a review system with written reviews.
Proof of purchase.
You download and can backup the files.
Updates arrive fairly quickly.
They have give aways on occasion.
Orbx
It’s mostly scenery.
They have a lot of scenery though!
It’s mostly pretty good stuff.
Downside. You get tied into an installer/manager system.
Upside. The installer manager system makes update handling really easy.
They always seem to have free stuff! Yay!