What makes you think we would get more with an universal roster? You are comparing an upcoming console with a handheld. The next console will probably be considerably more powerful then a PS3 whereas the 3DS is only above that of the Wii. These are completely different league of power and the 3DS will never be close to the Wii-U in power.
hypothetically let's say the forbidden 7 were actually in brawl, this would bring the character slot count up to 41. Since you said yourself the 3ds is more powerful than the wii, there is no reason there would be less that 40 slots
Why does it have something with what you say? Because what I'm saying is that with an universal roster, Sakurai will be much more likely to neglect the main concept of Smash; to play with Nintendo characters from your childhood and to be able to smack other Nintendo favorites as well. Childhood memory don't consistent entirely of Mario, Pokemon, and Zelda, they can also consist of Earthbound, Ice Climbers, Golden Sun, and Starfy, maybe even Sin & Punishment if you lived in Japan. If we get exclusives on the Wii-U, Sakurai will have no excuse to do it and as such, we will almost certainly get a mid-40s slot roster on the Wii-U. Plus most people are going for the Wii-U version, not 3DS version.
And you say that we would get 50ish characters with an universal roster? You are setting yourself up for disappointment. The chances of that happening is probably almost non-existence. Also, my reason for saying we will end up with 40 or less character slot roster is more then just pessimistic behavior, what I provided shows that Sakurai will more likely then not disregard the benefit of properly compressing the game.
why do you assume he won't compress this time. Post other examples of his games that show he has a habit of not compressing. Otherwise this could just be a result of time constraints