The asset that backs final settlement is different. I can always get cash from some other source to cover scenario 1 (which is also a more straightforward scenario because of that). A share is not some magical, fungible thing that is equivalent to cash; it represents an actual share in a company which comes with actual, legal rights. If a day comes where, as a completely hypothetical example, A is unable to deliver the share they promised to B then there is no externally resolving asset to settle the trade (unless parties agree beforehand to compensate in cash, in which case you get a functional equivalent to scenario 1).

While the mechanics are more-or-less equivalent, the settlement assets in the two scenarios are not which can make a big difference depending on the circumstances of the trade. Neither is generally a problem in a high volume, liquid system though.