I'd look at the native aspect ratio of your final output, and base your project on that (when all else fails, do a test render both ways, and look at how it appears in full screen mode on the website/file format). As bironic mentioned, the double letterboxed 4:3 can look incredibly odd.
Never never never never never stretch and squeeze, it makes a Greyias cry.
If you go with 16:9, for your 4:3 clips you could lay them on top of another video track that either has a pretty image or a motion back, or something of that nature to take up the space. It's not perfect, but it takes a lot less time than panning/scanning. Depending if you're working with HD footage, or SD widescreen, that method might technically be blowing up your footage.
no subject
Never never never never never stretch and squeeze, it makes a Greyias cry.
If you go with 16:9, for your 4:3 clips you could lay them on top of another video track that either has a pretty image or a motion back, or something of that nature to take up the space. It's not perfect, but it takes a lot less time than panning/scanning. Depending if you're working with HD footage, or SD widescreen, that method might technically be blowing up your footage.