After encountering a problem with my ZAT501HD I was offered an "upgrade" to the 502 by Centronics. I have to say that it is somewhat of a mixed bag as to which of those two is better.
ZAT502 v. ZAT501
+ better tuner (Samsung module vs. Thomson in the 501) gave me a few more channels
+ HDMI passes audio (the problem with my 501)
+ Dolby (but apparently only on the optical SPDIF out - mine only passes PCM over HDMI)
+ simplified setup and menus (the 501 was overly complex and sometimes undecipherable)
+ S video output
- cannot delete channels !
- no favorite channels function
- less detailed on screen guide
- no timer function (although I never used it on the 501)
- no VGA display port
- no serial port for software updates
As to it's screen filling zoom abilities, if a station is transmitting HD resolution none of the zoom modes will remove the left & right bars on 4:3 program content with the Screen Ratio set to 16:9.
So, unless your display has a zoom mode that will do it you will need to configure your ZAT502 Screen Ratio to 4:3. Then "Center" mode will stretch and remove the bars on 4:3 ratio SD program content and "Full" will provide proper screen fill for 16:9 HD content.