In terms of design, Otamaro looks more like a baby tadpole. It definitely appears to resemble a human head wearing headphones (hence its sound theme).
As with Poliwag, yes it has the hypno spiral but I can't appreciate its overall appearance that well. It is quite obvious that Poliwag is just a "generic" tadpole with a spiral on its belly.
I'm sure many fans are voting for Poliwag for the nostalgia factor, but in terms of actual appearance, I just seem to appreciate Otamaru's design a lot better.
However, Otamaru's evolution is so "derpy" that I would prefer Poliwag's evolution (Poliwhirl) over it in appearance.
My votes: Otamaru > Poliwag, Poliwhirl > Gamagaru, and Gamageroge > Poliwrath.
Of course, this thread is comparing Otamaru and Poliwag. I am one of the few who will ignore the nostalgia factor for the overall theme. Otamaru is my winner here.
Maybe if Poliwag's design was a little less "generic" (round tadpole that just grew legs with hypno spiral on its belly), I would have picked it.