I played Wii TP first and I've always felt that TP was easier. However, now I'm wavering. I was gonna keep this post short... but reading some of the other posts gives me a need to break it down.
Main Puzzles
Both Twilight Princess and Skyward Sword had really obvious puzzles. Even if you did get stuck, your partner could help you by giving obvious clues. However, I think the award for easiness here goes to SS because it had the biggest variety of help options. There were some puzzles in TP that even Midna would not help you with, but in SS you could always ask Fi... or a gossip stone... or dowse...or press 2...etc...
Side-Quests and Mini Games
Both TP and SS had easy to find, easy to figure out, easy to do side-quests. Again, SS had a variety of ways of finding and TP gave you the fortune teller. The generally agreed on challenging ones are the TP Cave of Ordeal and the Lanryu Boss challenge (on Hero Mode for me). The mini-games in both games vary from easy to medium. However, I think we all can agree that the TP roll game is PURE EVIL HARD! (And by "we all" I mean "me, myself and I") And it is because of that obviously difficult game that I say SS is easier in this category. While you can argue which mine game gives what kind of challenge, the roll game wins.
Enemies/Bosses
TP gives you the power to cut through your enemies with 75 million different ways to do it. All of SS enemies require timing of the Wii-Mote and/or nunchuck. TP's bosses all had an obvious weak points and easy to figure out, easy to accomplish strategy. SS's bosses had obvious weak spot with easy/medium to figure out, easy/medium accomplish strategy.
Yeah, TP is easiest in this category hands down.
Treasure Finding
TP had a fortune teller, but then again, so did SS. You know what TP didn't have? Dowsing, gossip stones that talked, a map that showed where all the chests were in the overworld, and various amounts of ways of finding your treasure. TP provided some rather interesting hiding spots for their treasure including many underground caves. SS had some interesting places as well, but all that goes out the window when you include its various ways of telling you where they are. SS is the easiest in this category.
Wow, I honestly thought (and still kind of do think) that TP was the easiest of the two. But breaking it down proves otherwise. I guess I'm not sure where I stand, but looking at these categories it appears that SS is the easiest.