I adore both of the shades. Mermaid's Dream is definitely the more memorable of the two, but it's an extremely bold statement polish and I find it a little overwhelming to wear alone on all my fingernails. As an accent on a nail or two, it's beautiful and unique. Conservative folks will probably want to stick to On the Beach because it is bright yet simple. It looks great on both the fingernails and toenails.
Mermaid's Dream is truly unique polish in terms of color and texture, a departure from her traditional polish with hexagonal glitters. It is reminiscent of the ocean, of a beautiful mermaid's tail. The base shade is a slight frosty seafoam green and the glitters are a mix of gold and blue microshimmers with larger round blue particles. The formula is thinner and creamier - smoother - than her usual glitter polishes, making Mermaid's Dream much easier to apply. I only used two coats on my ring finger because I was in a rush, but if I were to wear the polish on all of my nails, not just as an accent, I would probably use three coats. It requires a topcoat for glossy shine and longer wear, especially if you prefer nail polish that is smooth to the touch. The blue and green tones mesh so well together, yet the colors don't look overly blended together. I think it's the gold that adds balance with a whimsical touch! Beautifully done, one of my top 5 DL polishes.
On the Beach is considered a riviera blue. It is most similar to YSL's Summer 2011 #138 polish, but On the Beach is lighter, brighter. I think it's an ideal Spring/Resort shade. It isn't an office-appropriate shade, but it is quite fun! It brightens up my tan without making my skin look sallow, and it has a slightly watery, jelly formula. It applies perfectly in two coats, however, quite glossy on its own but the photos above are shown all the fingernails topped with her Addicted to Speed topcoat. Some have asked how close On the Beach is to Butter London's Slapper, which is my favorite polish from their Spring 2012 collection. They are not similar at all. On the Beach is a true turquoise while Slapper is a true teal. In fact, Slapper actually pulls more kelly green against my skin tone than teal. Which do I prefer more? BL Slapper, because it's so deep yet so bright, and I don't have anything else like it.
Bottom Line: I really cannot say which one I prefer more. I had to have both, especially because they pair so beautifully together. If you're on a budget and can only splurge on one, I'd stick with Mermaid's Dream.