I've not watched Best Mistake but having seen the trailer I can say that Be My Boyfriend is far superior - not only because CLC does one of the OST's for it (which is brilliant) but the cast is great and the 9 minute run time keeps you wanting more. In fact, that's it's negative too as I'd love it to be a full drama.
It's a guilty pleasure - I've rewatched the series each week waiting for the next episode. Lee Si Woo (Bingbing in Sisyphus) is the star of the show!