What are the "GDR" vs "QFE" hotfixes for SQL Server 2008 R2 SP3?
To understand the jargons, Aaron Bertrand has a #BackToBasics blog post



Also, refer to this decision matrix for TLS 1.2 support or MS KB - TLS 1.2 support for Microsoft SQL Server

10.50.6220 => SQL Server 2008R2 SP3 GDR 

10.50.6529 =>  SQL Server 2008R2 SP3 QFE

From the decision matirx, if you dont want TLS 1.2 support, dont install 10.50.6542. TLS 1.2 support got introduced in 10.50.6537. 

So to answer your question which one to install ... it will be 10.50.6220.

As always, test out your application behavior when installing a new fix.