The Digidesign 192 converters are the same converters as the 896HD. There is no sound quality difference, in fact the analog circuitry sounds better on the Motu, in my humble opinion. Of course the mic pre's on the 896 are lifeless, but then again the 192's don't have them at all.
With TDM you can use the TDM plug-ins as well as the Logic plug-ins via DTDM.
I personally do not like using PT hardware with Logic. I don't like the PT plug-ins and there is no noticeable difference in latency with native hardware set to the fastest buffer setting (assuming your also not trying to run mastering plug-ins while tracking).
What I really don't like about running TDM is the limited nature of using multiple systems. Instead of having 1 PT system, I can have several Logic rigs. I can have several people working on a project at one time. I can load my session on a laptop in a hotel room without any additional hardware. What more could you ask for? TDM is not like CoreAudio. You cannot load a TDM session into a native rig without having to do a bunch of session changes. With CoreAudio I can have different brands of interfaces (or just built-in) and load/edit/program/mix without having to change a thing from system to system.
My suggestion is, unless you need PT, buy yourself a laptop instead. You can always use it as a node if you need more horsepower.
