Differences you see between NG & TZ?

TZ: Serling controlled
NG: Laird controlled

TZ: black and white
NG: color

TZ: broad fantasy i.e. weird tales of all genres
NG: mostly horror oriented tales

TZ: closing narration to stories
NG: stories end with horrific image or reaction shot

TZ: one story per episode
NG: usually multiple stories

TZ: mostly filmed at MGM
NG: filmed at Universal

The ratio of good-to mediocre-to bad episodes is about the same for each series but a good TZ stood alone while a good NG tale could be marred by a weak story preceding or following it. TZ just seemed to stand out in the popular culture (helped by endless reruns and the annual marathon) while NG, like most shows of this type, simply faded from the scene. I think most fans today were kids in the late sixties/early seventies and NG scared the crap out of them. Director Guillermo del Toro (born in '64) is a good example of a typical NG fan.

Night Gallery wants most often to tingle the spine, whereas Twilight Zone was Serling's attempt to make statements about issues like injustice in society using Sci fi and fantasy as vehicles for his messages. And he did extremely well in that regard, although, there were times when it came off a bit too obvious and overstated. So, just because it wanted to make these grand statements, I do not automatically place "TZ" above "NG" in terms of quality.

The big differences are the writing - less Rod Serling scripts,
and the music - No Bernard Herrmann or Jerry Goldsmith scores.

I've watched maybe 10 episodes in this current marathon, none of which i remember, skipped several that i do remember clearly, a couple things that strike me are the bleakness of many episodes, not that that is such a bad thing but in a episode after episode marathon gets a bit much. also so many episodes have for lack of a better term a sort of "British" Hammer films feel about them.
all in all still a lot better than watching a Celebrity Bowling Marathon...

Twilight Zone was entertaining. Night Gallery was not. Or to put it in academic terms, TZ gets an A+, NG a C-.

for my part TZ holds up exponentially better...

I don't know if it's the 'Button up 60s' vs 'the dirty 70s'... or the 70's style creeping into supposed period pieces; or if the B&W format creates a distinction/delineation... maybe it was the Family not having a Color TV until I was 10, the made the B&W feels more natural to me than the Garish 70's Plaids and oranges that were exposed when I can see it in color (I remember when I had an old CRT TV (hand me down) in HS that I could fiddle with the settings and make it black and white and some of those 70's reruns worked better, and watching until too late in the am it was dimmer and easier to intentionally fall asleep to

but I've found that a Lot of shows pre 1965 (when Color took over) hold up better for me than the hair/fashion on display in the 1970s and 80s... give me Perry Mason and the Twilight Zone over McCloud and Night Gallery any day...
- Columbo gets a pass because Falk is epic, and the Columbo 'Movies of the Week' ran long enough to give me a point of reference

If you really want a good idea of the differences between TZ and NG (minus all the bilge spewed forth from some of the respondents here, at least one of whom has nothing more constructive to offer than "Twilight Zone entertaining, Night Gallery not") might I suggest getting your hands on a copy of the book by Scott Shelton and Jim Benson, "Night Gallery - An After-Hours Tour?"
Shelton and Benson did an incredible job researching pretty much every aspect of NG. They discuss each segment of the series and give each segment a rating as well as a summary of the plot of each segment.
They discuss the differences between the Twilight Zone and Night Gallery as well. Reading the book gives you the added benefit of getting impartial and informed opinions about Serling, Laird, the Twilight Zone and Night Gallery, instead of the biased and ignorant views espoused by a few of the people on the message board.
Happy reading!

Also, TZ seemed to have a higher calibre of actors.
And the look of a lot of NG episodes were experimental.