Seems NG and Gemini are sometimes releasing the same livery. Any have opinion on which 737 mold is better, and why? I am presuming here they are not all using the same common mold in their production lines.
Depends on the series.
IF you ask for the recent NG and GJ releases on the 800/900 series.
NG is way better; better mould, better printing, better details, better box art.
For the 300 to 700, take a look at Panda.
And for the 200 series, I'd go for Aeroclassics.
GJ's 737s are decent, but not as good.
You've asked a very simplistic question and I suggest you keep an eye out for reviews and photos from collectors to help you make your decisions. Check out my mould reviews section to get a better understanding of the different moulds that have been used over time. Here's the page for the 737NG:
Here's a video review of the NG 73H, which includes a quick look at competing moulds:
and a similar effort for the NG 73G:
When you take all factors into account the best moulds for each 737 type are as follows:
B737-100: No active moulds but old SMA is best
B737-200: Aeroclassics current mould
B737-300: Panda Models (also used by Aeroclassics)
B737-400: Panda Models
B737-500: JC Wings / Gemini Jets
B737-600: Panda Models or NG Models
B737-700: Panda Models or NG Models
B737-800: Panda Models or NG Models
B737-900: Panda Models or NG Models
B737-MAX: Gemini Jets/JC Wings or Panda Models
Obviously some think I have an agenda and they are right - it is good 400 scale models