Which NJ chips are you talking about? NJ offers 2 high end types of chips: ceramics (Desert Sands and Dead Mans) and traditional clay chips (Nevada Jacks and Martini Club made by Blue Chip company). Both of these are real casino quality chips but have vastly different feel and sound aspects to them.
As for comparing them to Nexgens, Nexgens are toward the high end of cheap plastic, metal slugged chips while the NJ are casino or near casino quality chips. Trying checking out
www.chiptalk.net; there are some detailed reviews of all 3 types of chips.
As for which ones are better? Depends on what you are looking for. The NJ chips will stack better than the Nexgens are somewhat more secure as they are more expensive than Nexgens. Nexgens appear to be the chip of choice for custom labels while the NJs only come in denominated chips. Both chips are more than adequate for their primary function of tracking how many $$$ someone has won/lost so far.
Whatever you decide, I'd recommend you get samples before you pull the trigger.