It's tough balancing them. I go through phases where I play my DS (or the female's DS) and then I go through phases where I play the Wii. The two hardly ever overlap. It's really an impulse thing. "I'm going to play Super Mario Galaxy now." And then I do eet.
To be honest though, handhelds are much cheaper than home consoles, there's less hassle with getting into the main game and you can play them wherever. When I want to play a Wii game, I've gotta go into my living room (making sure the telly isn't being used.) Turn it on, get the disc into the tray, then wait for that to load up. It just always seems quicker with a handheld for some reason. Plus, you can use it on your bed, living room, outside somewhere. If I had a busy lifestyle, I think a handheld would be much more of use to me than a Wii.
I used my DS lots during my first year at uni, in halls. Then, in the second year, I bought a Wii for my house (when I moved in with the ex, who wasn't the ex at the time.) Then, moved into a different house, kept the Wii downstairs so everyone could use it, then used my DS in bed.
Playing the DS on the train though with Ine is really fun, though. And, it's nice to play even if you only have a few spare minutes of time. When she had a tutorial recently at her Uni, I waited outside playing Tetris.