I used to use 4:3, because that is the aspect ration of my camcorder. Now a lot of clips come from videos that I shot with my iphone, which uses 16:9.
Maybe in the future 16:9 will be the standard for video and photo cameras, smartphones, and websites like youtube and vimeo?
What are you using? Which aspect ration will be important in the future?
I use vdmx to make visuals for my band. We are just two people, so we don't need a big stage - 4:3 aspect ratio for the visuals behind us fits well.
And we upload videos to vimeo and youtube, where 4:3 is used (please correct me if I'm wrong).
Peter