Well, I grew up with the 80s versions of the characters, so I naturally have a strong preference for them. Except for Brittany, maybe - her overtly 80s design just seems painfully and pitifully dated today, lol.
I don't mind the current ALVINNN designs at all, and in fact, I really love all of the little details they put into their designs that help visually define their personalities, like how Alvin's jersey has some fraying and distressing on it, and not only that, but those little details help distinguish them better from each other. There are some minor differences I would personally make though, like giving Theodore something different than a hoody to wear, and Jeanette is way too skinny.