I used to vastly prefer 4:3 but that was back when I had an older TV. (Widescreen seemed so damn annoying because of the bars that it adds)
Nowadays I don't have a preference. Both are fine with me, so long as the proper hardware is used to display it.
For example, when I watch videos on my computer, I will prefer 4:3 versions over 16:9 unless on the laptop, when I prefer 16:9 (my laptop is bork atm ;o;)
Anyways, if all I can get is 16:9 then I will just watch the video in windowed mode (unless it exceeds my screen resolution, then I will view it fullscreen and use MPC to apply corrections to the video frame as needed)
I have a 16:9 TV I can use, so long as the file isn't in a Matroska (MKV) container, I can watch it on my TV via the Wii (Yaaaaaay Wii homebrew and MPCE n.n)