If you use QoS and STI together, the QoS component will ONLY capture the Back End Service Time, whereas a QoS listening policy is normally configured to also capture Round Trip Time and Page Render Time.
As I see it, the reason for this is that the STI component initiated the transaction, and that component already records round trip time, and can be configured (via the use of the Page Analyzer) to capture VERY detailed information on round trip and page render time.
I see absolutely no problem with this if you are only using the ITCAM GUI to access the data. However, if you're using another tool to access the performance data (Managed Objects Formula, Crystal Reports, etc.), this can be a problem since Page Analyzer data is not stored in the database. Rather, this data is stored in individual files on the Management Server (and there is no command-line access to these files).
To me, this still isn't a large problem, since you're normally much more concerned with "real" user transactions. I just wanted to document this caveat.