In short, there is no difference between the two since they are mutually exclusive.The H/W Buffer Setting within Pro Tools (Playback Engine) only applies to RTAS. In addition, the RTAS processing engine is completely separate from that of the StreamManager engine (which is used for HTDM and TDM DirectConnect). The 128 StreamManager Extension is the only way by which you can set the StreamManager buffer size to 128 (there is no user interface setting for the StreamManager buffer size because many DirectConnect apps would be unhappy with a sudden buffer size change while a session is open). The result of using the 128 Stream Manager (instead of the default Stream Manager which uses a 512 sample buffer) is lower latency, but the byproduct is more CPU demand.The buffer size of StreamManager does not affect the RTAS buffer size. Likewise, the H/W buffer setting in the Playback Engine doesn't affect the StreamManager (HTDM and TDM DirectConnect) buffer setting.