Is there any difference in regard to QoS between IPv4 and IPv6?

This is an interesting one that comes up very often. Basically there is no difference in the concepts of QoS for the two protocol versions. The only difference lies in the fact that the headers are slightly different so different fields or headers (extension headers) may be used. Refer to RFC 2474, "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers" for an example. It specifies the DS field for both versions of IP.

The following fields and headers from the basic IPv6 specification can be used for QoS: Traffic Class, Flow Label, Routing Extension Header, Hop-by-Hop Extension Header