Yoshi's Story is one of the simplest games in the world. You can literally have this game beat in about 40 minutes, probably half that.
For those who have never played this quirky little game, Baby Bowser has turned Yoshi's Island into a picture book and stolen the Super Happy Tree (yes, that's really its name), and all the Yoshis decide that ain't gonna fly, so they set out to confront Baby Bowser and set everything right. To do that, Yoshi must nom him some fruit in each stage. Eating thirty pieces lets you move onto the next level. That's all there is to it. There are six worlds, so to speak, and you visit one stage in each world. By finding giant, smiling hearts, you can unlock up to three new stages in the world you're currently in. You'll have to replay the game to play the stages, however.
Difficulty-wise, this game ranks a zero out of ten. Barring you falling down a hole to your death, it is very, very difficult to die in this game. You even get massive invincibility against the final boss.
That said, Yoshi's Story can be one of the most difficult and challenging games you've ever played, if you let it. In each stage, in addition to the oodles of generic fruit you can find, there are exactly thirty melons. Some of the melons are in plain view, while others require you to literally sniff them out. The most difficult part of the melon hunting, however, is the mini-game found in each stage. Each mini-game has a maximum prize of seven melons if you complete it quickly. Dawdling will reduce your maximum prize, meaning you have to be quick.
Some of the mini-games are downright difficult to win at, and you get one shot at doing it. If you fail the mini-game, or you don't get all seven melons, you're screwed for getting all 30 and you have to either give up or kill yourself to try again.
While this game has cool graphics, awesome music, and a pimpin' black Yoshi, the melon challenge is the only thing that makes Yoshi's Story worth playing if you're over the age of five. If you don't have the patience to do some serious hunting and not throw the paddle when you fail the mini-game--again--you will feel like you've wasted ten bucks.
Paper Mario, on the other hand, is a super king game I have no problems with. If you like Mario and RPGs, Paper Mario will be right up your alley and you'll spend a heck of a lot longer than 40 minutes beating it. Paper Mario is more complex than Yoshi's Story could ever be (though that's not saying much), and you'll definitely get your money's worth should you choose to play it.