Nice roster, Jazzman.
Anyway, here's my idea for version differences...
3DS Version
*Same amount of characters as the Wii U Version, yet the characters have lower-poly models and don't have alternate costumes like the Wii U Version. Basic play-style is the same as the console version.
*Bottom screen in the 3Ds version displays hit points, the current score, and other info that would usually clutter the top-screen.
*3Ds version has a few items and Assist Trophies that aren't in the Wii U Version that take advantage of 3D.
*Less levels, and different ones, compared to the console version. Levels in the 3Ds version are designed around the 3D effect. The music selection is also different compared to the Wii U version, with only 2 songs per stage and you select the music using the L/R trick from Melee. All the stage songs in the 3Ds version are composed exclusively for it and aren't found in the console version.
*Multi-player modes, local and on-line are included, with less options than the console version, but focus is on single-player and practicing for the console version. Online isn't as good as the Wii U version but decent. Only supports 2-on-2 over WiFi (but can have 2 CPU players) due to system limitations, but local can be 4 players.
*Contains a simple Classic Arcade Mode and Story Mode, done in the style of the Subspace Emissary yet focused more on Nintendo characters and locales. The 8 Original Smash Brothers (minus Yoshi, replaced with Pit) each have their own seven-stage story with a mini-boss and final boss in which, on the way, they team up with other characters. Supports 2-player co-op off-line and online. There is also a 50 level Event Mode, single-player only.
*No Trophies, but you collect Stickers in the 3Ds version found randomly in fights and in story-mode. You can use them in the 3Ds version, yet you can also transfer them to the Wii U version, which has a new mode where stickers are allowed. Kind of like how there are NeoGeo/Arcade King of Fighter games with a similar power-up system for the main game. Stickers can't normally be found in the 3Ds version.
*Your accomplishments in the 3Ds version unlocks trophies in the Wii U version. Basically, apart from extra trophies and the sticker mode, you aren't forced to buy the 3Ds version. It's more of a single-player game you can use to practice your skills for the console version.
*Break the Target, Home Run Contest, and Board the Platforms, returning from N64 Smash, are only in the 3Ds version since it focuses on single-player.
*Would come out a month earlier than the console version so players could practice.
Wii U Version
*Same characters as in the 3Ds version, but with better models. They also have alternate costumes as opposed to simple color changes included. You can also use the 3Ds as a controller if you're more used to using it from playing the handheld.
*Some items and Assist Trophies are different from the 3Ds version.
*Way more levels compared to the 3Ds version, including a few classic levels, and most of the levels in the Wii U version are more focused on providing a fun multi-player experience while the 3Ds version has more gimmicky levels. The Wii U has multiple music for each level you get by collecting discs.
*Huge focus on multi-player, with many different modes to play including on-line tournaments, and basically the entire console version focuses on the multi-player aspect. The 3Ds can be seen as the "casual" version while the WiiU is sort of the "tournament" edition.
*No Story Mode, but a basic Classic Mode and All-Star Mode is included. There is also a Boss Rush mode where you battle Nintendo-series bosses, made for the game exclusively and different from the 3Ds' selection of bosses, which provides a challenge. Event Mode is included, with different events compared to the 3Ds version, and there are 50 Single Player ones and 50 Multi-Player Events.
*Hundreds of Trophies included obtained by completing certain events, including linking with the 3Ds version.
*There are, for casuals, a selection of mini-games included that utilize the Wii controller, similar to the Kids Club in Pokemon Stadium.