When aluminum frame is definitely much better not only strength-wise but also for water resistance, I found that my Fleetwood was build by "minimum wage welder".
Not only the welds were very poor quality, but not much of them and they kept on breaking, so I had to add steel reinforcement on joints.
Bottom line, it all comes to person who does final assemble and quality control.
To answer technical dilemma - aircrafts are generally exposed to high vibrations and guess what most of them are build with.
Few accidents in last decades show that it is steel in turbines who gets fatigued faster, than aluminum airplane body.
