Always "?!". I don't know that it's that big of a deal which one you choose, but I settled on that one a long time ago and have stuck to it ever since. Always the same for the sake of uniformity, even if it's a bubble with nothing but the raw "!?" I change it to "?!".
Mmm, I see cmertb ninja'd me, yeah, that most of the time you'll see "!?" used in Japanese is actually (as I remember) part of why I settled on "?!" for the English version. Japanese is read right to left, it makes a strange kind of sense to me to flip them to reflect English being read left to right
(yeah, don't read too much into it, as I said I think it's as much personal preference as anything