Tough decision for me, honestly. Both idol groups have their strengths and weaknesses both character-, music- and lore-wise, but I'll have to go with the Squid Sisters on this one.
Now, to explain how I came to this verdict, I'll write up a bit of random rambling that may or may not make sense to the reader, but here goes.
The Squid Sisters are like a good classic. They follow you throughout the Story of Splatoon 1, they broadcast news, stages, and Splatfest announcements, they're ever-present in their little studio accessible on the balcony we're all still missing dearly in Splat 2, and nobody will ever forget that epic moment when they did the music override in the final boss fight. Calamari Inkantation is arguably one of the best tunes the franchise has ever brought to the light of day, radiating with so much power and damn, it's so catchy, dont'cha think? As a group, they have the classical J-Idol vibes going on: Quirky outfits? Check. Funny dance moves? Check. Boke-Tsukkomi character dynamic? Check! Catchy J-Idol inspired tunes? Check. Their music resonates a lot with me despite being more on the Rock/Metal side of things. Listening to their Splatfest Plaza music makes me remember all the good old times I had in S1 and despite only having had one track for Splatfest battles (if we ignore the 1-minute now or never track), I never got tired of it. Playing even the saltiest of Splatfest sessions felt worthwile with the atmosphere being well-rounded through and through. Now lore-wise, they're both agents, cousins and aid you on your quest to return the great Zapfish and sorta prove their battling capabilities with that in mind (bonus points for Marie wielding the old-school Hero Charger in the boss battle of Splat 2) and you learn quite a bit about their past and future, giving them so much background story to work with. Their break-up on the transition to Splatoon 2 made the stress they've been under very realistic and even a bit sad, and Marie's sadness and references to an Enka singer in Splatoon 2 further underline this aspect. Also, I never grew tired of Marie's sass, and her being a charger wielder only added to the sympathy bonus going on.
Now Off the Hook are great in their own right. They have lots, and lots of contrast going on, design-and characterwise. Pearl being vertically challenged, pale and showing white "hair" while Marina is dark-skinned, an Octoling(!) and is overall darker, and tall even by octoling standards; Pearl being your bratty rich kid with a heart of gold and Marina the wallflower with a side of snark and tech-enthusiasm; Pearl being unable to sing and hence relies on rap, while Marina is blessed with a beautiful singing voice and overall both of them giving off the Western R'n'B + Rap vibe are a good addition to the cast. You can clearly see how they're very close friends throughout their banter which makes them interesting and engaging to learn more about, but this is sorta where they fall short, lore-wise. You learn so little about them. Heck, why didn't Marina get more story as to why she's an idol in the first place, being an Octoling and all? They wasted so much potential on both of them, which is really sad, considering they make for interesting characters as-is and their music is very good as well. They don't just have one Splatfest tune, but two or three even?! And all of them are quite nice songs too, sounding more "English" as far as Inkling can go there, lol. I certainly like them a lot regardless, I just wish they had more story going on, it also doesn't help that you have to mash through the buttons to get past their announcement as they now announce not only two, but three modes AND Salmon Run when it's up. While I like reading their banter, sometimes it feels like a hassle, y'know?
All in all, if I could just go "I like both", I would've opted for this option instead, but as I have to make a choice, ultimately the Squid Sisters resonate more with me, but that's really just my personal, subjective view.
