I'd say absolutely don't stretch/squeeze. My first choice would be to actually leave as is (um, maybe that's the letterbox option?). I've watched vids like that where some are shots are 4:3 and some 16:9 and it looks fine to me (the sources are different anyway).

But cropping should also work fine.

As a vidder, I'd probably pick which was easiest and which required the least work (if I have more of one size than the other, make them all the size I have the most of)