It's usually Nintendo's style to make games for one console alone (Twilight Princess being an exception since it had been delayed to the point of almost time for the Wii anyway). So yes, you will need a Wii U and 3DS for future Zelda games.
However, aside from Ocarina of Time 3D, which is a remake of a game you can get on the Wii, there are no Zelda games made or announced for the 3DS or WiiU. So at least you'll have time to save up some money to buy these consoles. I would suggest getting the 3DS first because generally handheld games take less development time, and with a console Zelda game coming out soon the next game will probably be for the 3DS.
Also note that many stores such as GameStop have trade in programs where you could sell your current DS, which could make some money towards the 3DS. Since the 3DS can play normal DS games (I'm pretty sure), you would have no reason to have both unless you are sentimental towards it.