Here is a shot of them in their bottles and from here they pretty much all look identical to each other.
They are all very similar with the green base and the glitter composition but there are no actual dupes here just a very slight variation on a theme. The glitter is about the same in all of them but what makes them different is the base.
For the first coat Julep had the lightest most sheer base and also the least amount of glitter, Sinful had the darkest base color, and Wet n Wild was about the same as Juleps for the base color but had the densest amount of glitter. After two coats Julep and Wet n Wild look the most alike still sheer base but the glitter is almost the same. Sinful colors is almost opaque at two coats.
After three coats it's like Goldilocks and the three polishes: Julep is still just too sheer and thin, Wet n Wild is so dense with glitter it is goopy but still not quite opaque, and Sinful Colors is just right with complete opaqueness and amazing glitter! In pictures the visible nail line can be seen on Nail Junkie but it is not visible in person.
So they are all similar but just a little bit different. Julep is at the bottom of my list because at $14 a bottle it should be so much better but I can get the same effect seen in the bottle from Sinful for only $2. Sinful is definitely the best of the three for coverage and price. The pictures below are shown with three coats of each polish and topped with HK Girl topcoat.
From left to right: Julep Harper, Sinful Color Nail Junkie, & Wet n Wild Happy Holidaze!