I've used both steel and aluminium for touring. After many tours and many miles I have realised the best thing to do is just use a decent bike that does everything you want from it, and not get too concerned about which material is the best, because most good frames will do an excellent job.
Even though I do have a slight preference for steel, I wouldn't mind either one actually.
Magazine reviews with often mention steel has the advantage over aluminium for touring, because steel can be fixed/welded if part of the frame snaps or breaks whilst abroad. Even though that is true and relevant, how often does that sort of thing happen? There must be thousands of cycle tourists on all types of bikes cycling parts of the world... only a few will prang their bike to the extent of breaking it. So if there's an aluminium tourer that appeals.... I'd say go for it.
Cannondale make some superb Aluminium tourers; I've owned a couple and they feel fantastic to ride. I've also toured on steel Dawes Galaxy's and the like - all are just fine.
So yes, I'd plump for steel myself, but it's definitely not a night and day thing. Cheers. Mark.