I’ll start with specyfications of both models and then I’ll give few words from myself:
J2
System: Android 6.0;
Screen: 5″ 1280x720px, SUPER AMOLED;
CPU: Spreadtrum SC8830;
RAM: 1.5GB;
Storage: 8GB + microSD;
Cameras: 8MP and 5MP;
Battery: 2600mAh;
Other: Smart Glow, TST;
Size: 142.4 X 71.1 X 8.0mm;
Price: INR 9750, so around $145.

J Max
System: Android 5.1;
Screen: 7″ 1280x800px, IPS;
CPU: probably Spreadtrum SC8830;
RAM: 1.5GB;
Storage: 16GB + microSD;
Cameras: 8MP and 2MP;
Battery: 4000mAh;
Size: 186,9 x 108,8 x 8.7mm;
Price: around $200.

Ok, so now promiced few worlds from me… I really don’t know what to wright. Well ok, they are quite cheap but yeah… J2 have at least two new things.
If we’re talking about J2, I informed about this model here, we have less RAM then suggested but TST should mede for it, at least in theory. And what TST is you aks?
TST stands for Turbo Speed Technology – to put it simple techlogy for electronic device to complete task with as little resources and fast road as possible. It should shorten starting time of system apps for 40%. Is it working? We don’t know yet.
And now my big disappointment Galaxy J Max, as you can see it have three upgrades in comparison with J2: bigger screen, more internal storage and bigger battery. But with screen are two major problems – first it’s phone and this size is uncomfortable to handle when calling and size is bigger but resolution is same what means less piksels per inch. For me it’s rushed made simply to have responce for Huawei V8 Max and wrong way.

Source: Samsung via gsmarena and gsmmaniak (polish)