Mercury Meltdown is completely different from Kororinpa, it's slow and deliberately paced (due to being originally designed for joystick controls), with more of a focus on puzzles. Kororinpa was built from the ground up with tilt controls in mind and it shows, with a fast-paced arcade feel and level designs featuring much more demanding layouts and obstacles. You'll often be forced to tilt the entire stage 90 degrees to navigate your marble up walls, which adds a LOT of complexity to the game.
As for which Kororinpa is better: Marble Saga obliterates the original in terms of content, with more than 5 times as many available levels, plus a level editor -- but for some reason they decided to mess with the original's perfect physics, with near disastrous results. With the new physics, the marbles you have access to at the beginning of Marble Saga all feel sticky and unresponsive, like they're constantly rolling through molasses. Fortunately you can unlock more marbles relatively quickly which feel a lot closer to the snappy, responsive controls of the original game. Or, you can unlock the best marble at the start of the game by inputting a code, which I highly recommend doing. Once you get past this control issue, Marble Saga is easily the better game.
(poor Japan had to wait until October!)
