Kill la kill is strictly a popcorn shounen. It's got all the insane fights you'd expect from a shounen, but with all that lovable goofiness you'd expect from trigger. The experience you have with KLK is based heavily on who you watch it with, if you watch it alone and look at it critically it's pretty shit, but I on the other hand watched it with my friend and we just made dick jokes the whole time and loved it, though we agreed it wouldn't be worth a second watch.
Gurren, on the other hand, is a show that takes the formula i explained before, but multiplied tenfold. It makes the bold decision to kill off a main character (not spoiling who) early on, and from then on you feel actual tension in the fights since anyone could die. Plus the writers decided to up the stakes every single episode, instead of every season like most shounen.
In a typical shounen, what's usually at stake in a given fight progresses like this
I have to save my friends>I have to save my friends but now there's more> I have to save EVEN MORE FRIENDS> etc. until he has befriended the entire planet and therefore must save the world.
In KLK, it goes I must avenge my father> I must avenge my father and protect my friends> I must avenge my father qnd protect my friends and save a bunch of other schools> etc. until she saves the world.
Meanwhile, gurren's laughing because by about 2/3s of the way in they've already saved the planet like 3 times and are fighting the fucking moon.
What sets Gurren lagann and Kill la kill apart from the rest, though, is that they acomplish these things all in about 26 episodes, with other shounen going on for hundreds and hundreds. All in all, grab some friends and watch one of em, they're both solid, but gurren lagann is still leagues ahead
My therapist told me that I gamble in order to escape from the realities of life and I replied that's why everyone does everything. - Norm MacDonald