- price - yes, there can be differences
- reliability - no, MS / Asobo don’t test third party planes. They’re simply providing the marketplace for them
- upgrades - right now, yes. You can expect updates to be available from the developers websites sooner. I hope they will eventually get the option to inject their updated versions into the marketplace themselves.
What you didn’t mention, and could be a pro for the marketplace, is the licensing. Since your purchase is linked to your XBox account when buying from the marketplace, you shouldn’t have any issues with the product license. Not sure, but maybe that could be an issue when migrating to a new PC? not sure. Anyways, it’s nice to have all your content from a single source.