I own both sidesho and a 510. Get the Sidesho. I don't hate my 510's. They are nice products, but they're not great. The sidesho is reliable, has a great atty and is easy to use. The atty is great for dripping, (it's exposed) and the cartridges hold a pretty decent amount of liquid. I'm still using my original attys that came with my first kit in February. More proof than that? I think I quite literally own every possible battery color and atty for the Sidesho. They're really great. You also might want to take a look at the 902's. Larger, but still nice looking, great atty (though not exposed) and holds a ton of liquid. Also customer service from Tony is incredible! You will not be disappointed with Modern Vapor. Oh, almost forgot, shipping is super fast. If you have any specific comparison questions I'd be happy to answer them.
As far as flavor, definitely Sidesho, the atty doesn't burn some liquids like the 510's do.