Alright let's see.
DKC Returns on the Wii and 3DS was already a good game. For Tropical freeze, they up the ante for pretty much everything: The graphics are better (being in HD kinda helps), there are more controller options (So you don't have to use motion controls anymore), you can choose between 3 amigos (as opposed to only Diddy in returns). That's only on the technical side. Where TF really shines is in the level design. Returns had the standard tropes : The Jungle, the cave, the forest etc etc. Tropical Freeze just goes nuts : Riding on a minecart through a sawmill or on giant fruits? Swimming in grape juice, or while dodging a giant octopus? Platforming in the inside of a tornado, in a burning forest, inside a giant tuba while riding the air current? You got all of this and more! And the cherry on the top is the AMAZING soundtrack by David Wise, the creator of the original soundtrack for Donkey Kong Country 1 and 2 for the Snes. And yes, there are remixes of Stickerbrush Symphony and Aquatic Ambiance! I recommend checking out this video review if you want to know more.
Yoshi's New Island on the other hand is supposed to be a sequel to Yoshi's Island on the Snes and GBA. The original yoshi island is a classic, and easily one of the best platformers out there. Unfortunetly, New Island is only a copy and paste job. If it was only that, then I would say it's still a decent game. But it's even worse. Compared to the original, It's slower, clunkier, uglier and as a contrast to TF, the soundtrack is AWFUL. It's literally a single song remixed over and over and over again, and not in the right ways. The last thing that theme needed was a kazoo version, but it still exist. For the few things YNI does differently: The huge eggs doesn't add anything. It's just a screen clearing device. The transformations are different, and by that I mean they are downscaled from the original. The game is also piss-easy and boring most of the time.
If you want a Yoshi game, the original Yoshi's Island for the GBA is on the Wii U virtual console. Get Tropical Freeze!