The Z series is based on a MVA panel, its an EXTREMELY nice TV, very high contrast ratio, but colours will shift slightly at extreme angles(moot point when it comes to TV's usually). Its a very thin TV, with a nice brushed bezel. The XBR6 on the other hand has a slightly lower contrast ratio(very slight), but will not exhibit any colour shifting at extreme angle(much less than the Z series anyways). Its also a physically larger TV(thicker), but has a slightly better up convert engine(this is important if she watches lots of DVD's without an up convert player or regular TV channels alot) than the Z series.
Both TV's use the XMB crossbar as a menu UI, which IMO is a GREAT and easy to use UI to edit settings in and etc. FYI I'm a Sony fanboi, so thats why I know so much when it comes to the Sony. I don't know much of the Samsung TV's, BUT Sony and Samsung use the SAME LCD factory(they jointly built and run it) so the LCD panel quality is going to be the same in similar price/feature ranges. Because of this, its usually only features and appearances that separate Samsung's from Sony's.
I recommend the Sony overall, I've had great support from them when my 52" TV had issues, they have a 48 hour policy on their TV's, meaning that a TV technician will be out to contact you, pin down the exact cause, see/diagnose the TV within 48hours of the service call. They may not actually go to your house with-in 48 hours depending on if they can diagnose the problem over email/phone(in my case, I asked the support line for the Technion to contact me with email, they give you a chose, he didn't need to come over, a picture sufficed in his(proper) diagnose), but they will have the parts and etc on order and diagnosed with 48 hours of the service call. Even thou my part wasn't in stock, the technician was able to get it in in only 3 days, I was happy overall.
Take my opinion as biased if you like, as I am a self professed Sony lover

(FYI Sony Electronics, and Sony Entertainment are separate company, like how DC and NCIX are different, what one does is not the fault of the other *cough rootkit*, so don't blame Sony electronics for Sony entertainments doing)