It depends what type of town you are going for. Lily fits better in a Zen or Fairy-Tale town. A frog will always fit into a town with plenty of water-textures. For a town with lots of forestry, flowers and 'normal' PWP's I'd go for Poppy. Aesthetically, I find Lily more interesting.
I've had both Poppy and Lily, so I can speak from experience. You should go with Lily. Why? When it rains she won't use an umbrella, something only occurring with frogs. Plus, when she wants new furniture, give her a frog, the dialog will be absolutely hilarious (I won't spoil it for you, but you'll laugh when you read it)!!
Lily! And that's not because my name is Lily too, haha. IMO, Lily is the cutest frog and frogs are so cute when it rains... no umbrella~ and her cheeks have a pink circle AHH sooo cute!!
Go with Poppy! Sure, Lily is the cutest frog besides Puddles, but I'm just more of a fan of squirrels and their adorable swirly tails (cough Marshal cough) than frogs.