Well, having both are best. 4/2 Ninja Monkeys can hit up to 20 bloons at once (because it's 5 shurikens + Seeking Shuriken + Sharp Shuriken; 5 x 4 = 20), whilst 2/4 Ninja Monkeys have Flash Bombs that blow up Lead Bloons.
Ninja Monkeys need to be x/3 to be able to reach x/4, you know. And Flash Bombs don't suck!
In my opinion, Bloonjitsu is good in early rounds. A single bloonjitsu (with a bomb tower, of course) can hold many bloons. But on late rounds or strong rounds, SSL can be one of the most powerful towers as it slows down ALL bloons in 10 seconds including MOAB-Class!
However, I rarely use SSL because I'm tired of tapping the skill button all the time. DX
4-2 works well against spaced bloons while the other path is designed for popping grouped ones (not including the skill).
Why does my reply appear twice...?!
still the 4-2 especially if you are able to set up an MIB (x/3 Village) so that you can pop everything - that is lead of course. a 2-4 alone may not pop black or zebra, otherwise using an MIB won't make a great deal either
Just delete one of them.
4/2 Ninja combined with 2/4 Monkey Village will also rip apart Bloon rushes and smash through so many things. It's a shame that it is unlikely that such a powerful crowdstopper combo can be achieved in Regrow Rush of Death rounds in BMC before a huge swarm of Pink Bloons rush through all your defenses and quickly make you lose all lives...