I'm inclined to say J-cutter is better, but they are different tools with different uses.
J-Cutter it hits more targets, and you can use SNS to make your first hit special have normal accuracy(you must be at max range for that), minimizing the need for hit attribute. Also, what I like about it, is that it will hit the enemies behind the one you are targeting. Enemies walling your target is very annoying.
Kunai on the other hand will likely have more accuracy on single targets, a few things to know about cards is that the 3 projectiles of the 3rd hit do not lock on to targets like shots, so you have to position yourself accordingly in account for their angle. They can, however, all hit the same targets at point blank, greatly increasing chances for para to proc.
Cards also have infinite vertical range making them great to hit things straight above you like: Zus, Gryphon, Dragons, Dubswitches.
They are slower than slower and have less range than handguns, but faster and more range than slicers.
What is better? It depends on what you use it for, and your personal preferences.
J-cutters: AoE multi target, unreliable at close range/point blank.
Kunai: Range, multi-tool, semi-multitarget, drip
Rangun: Range, speed, accuracy but single target, more affordable, (could get very high hit for the price of the 2 other items)