If you really aren't going to use the Xbox, then go ahead.
Otherwise I would say that you're likely to find more games you enjoy on the 360 compared to the 3DS.
If you definitely want a 3DS though and won't miss your Xbox then go for it.
But be aware that Tomodachi Life and Animal Crossing can get boring fairly quickly IMO. They're novel, but if you get bored with routine (especially in AC:NL) then you might be wasting a purchase if you're not used to those types of games.
Might I also suggest you grab yourself a decent SD card of good size. A 16 GB SD card will let you hold a fair few downloaded games, should you purchase from the Nintendo eShop (which generally I don't recommend if you're trying to get good deals, since the prices are stuck up on high there).