I have mixed feelings for the both. Heart Containers or Pieces of Hearts, it really doesn't matter to me at this point. Also, with this question being made comes whether or not there should only be 16 Heart Containers total or 20.
Having full Heart Containers as a means of obtaining more life wasn't just seen in Phantom Hourglass and Spirit Tracks, but it was also seen in the original Legend of Zelda and the Adventure of Link. With Heart Containers scattered around, the game will be shorter, sure. However, with the Heart Containers, its highly possible that accomplishing the one side-quest to get one Heart Container will be more difficult than doing, say, four easy side-quests or some of medium-difficulty. Collecting just Heart Containers, as I said, was seen in LoZ, AoL, PH, and ST, and all these games had 16 Heart Containers total, with the exception of AoL. If it is only Heart Containers, then its possible that this game may as well have 16 Heart Containers total, which could possibly bring the difficulty of this game up.
Pieces of Heart didn't make their debut until A Link to the Past, and all the games after that had Pieces of Hearts, except for PH and ST, as means of acquiring more life. As much as I like more length in a game, I sometimes found it tedious, much like in the Wind Waker. The good thing with Pieces of Hearts though is that it may give some other items some more significance. If the side-quests for the Pieces of Hearts tear through supplies, such as arrows, bombs, etc. then they'll definitely bring up the value of a Rupee which normally they just lay in Link's wallet for a good amount of the game. Also, most side-quests I've finished for Pieces of Hearts are easily done. However, I'm not saying that Pieces of Hearts are always easy to collect. The side-quests for the Pieces of Hearts, or at least some, should have the difficulty of that seen in Majora's Mask with the Couple's Mask. However that's just my opinion. All Zelda games with Pieces of Hearts, though, had 20 Heart Containers total, with the exception of LA and OoX, which can make for an easier game unless something counters that like stronger enemies and the like.
As I've said, I have mixed feelings on the two right now. I would like to see full Heart Containers in the game to collect, but I expect that there will be Pieces of Heart.