Speaking of the Smash 3DS soundtrack, it feels a lot like a "Lite" version of what will be the Wii U soundtrack.
I'm pretty sure all songs featured on the handheld version will be on the Wii U version as part of the My Music feature.
Something that bothers me on the 3DS selection is... I don't really know how to explain it, so I'll just ramble along...
Pokémon music only covers Black/White and X/Y. Feels lacking without some music from older games.
All Kid Icarus music is from Uprising, so the same criticisms for the Pokémon series apply here.
Wrath of the Reset Bomb has two versions, one remixed for the stage and the original version as an extra. Two versions of Dark Pit's theme are also featured: the ground theme for the stage and the aerial version as an extra. This feels very redundant. Boss Battle 1 (original) and Inside the Space-Pirate Ship (remix) are the other songs.
Metroid really got the short end of the stick by only featuring the two Melee songs: Brinstar and Brinstar Depths. Likewise, Yoshi only has two songs from Brawl.
Kirby mostly gets 8-bit songs for the Game Boy stage. It also has the new Green Greens remix and Legendary Air Ride Machine from Brawl.
Fire Emblem, Xenoblade and Sonic don't have any new remixes, as great as some of the soundtracks from these series are, it still feels lacking.
Donkey Kong has Kongo Jungle from Smash 64, Gear Getaway from DKCR and the Brawl mix of Bramble Blast. Few songs, but there's variety here. Oddly enough, the Jungle Japes theme from Melee isn't in the game.
Let's not even talk about F-Zero, shall we?
The only series that I feel that have more robust song selections are Mario and Zelda, featuring songs from different games, both old and new. The rest of the series have kind of lackluster song selections...
Then again, I might be a little spoiled by Brawl's immense soundtrack. The Wii U version will likely stomp the 3DS soundtrack to dust.