You're misinformed about the nature of the SQL Server codebase. While it was originally licensed from Sybase, all parts of it have been completely re-worked. I read somewhere that they celebrated "zero Sybase day" or something to that effect in the early 2000s to mark the retirement of the last of the original Sybase code. Some more info here: https://blogs.msdn.microsoft.com/euanga/2006/01/19/sql-mythb...
As for its performance and features, I have run PostgreSQL, MySQL, and SQL Server in production on large workloads and am a big fan of PostgreSQL. That said, I've encountered the fewest sharp edges and disappointments with SQL Server. It is a very respectable RDBMS and it's great that it's now available on Linux.