For practicality probably the violin.
And no, it's not a difficult instrument.
The only hard part about the violin is that it's somewhat crucial to be able to play with a music file playing.
That's something I have a hard time with, but it's not impossible.
Makes me wonder; I should have probably not start playing the piano and just go for the violin. Not only can I stand with the instrument, it doesn't take much room in at all.
Also you can play and swoon around walking and stuff. Pretty neat eh?
Instead, I'm now practising a schumann, tchiakovsky, bach and chopin piece. Hurray!
Oh and did I mention I absolutely suck at tuning a violin? You only have to tune the piano 1 to 2 times a year, win win!
I'm being cynical over here btw.