Originally Posted by
cyccommute
Wait a minute...I have to get on the fire suit.
Go with an aluminum bike. One of the things I found when I got my Cannondale in 2003 was that I could ride out of the saddle with a touring load. It was about the first thing I noticed. I could never climb on my old steel touring bike that way. The frame would wiggle around and wander all over the road. If I did manage to stand and pedal, I had to do it by pedaling straight up and down without throwing the bike from side to side like you would an unloaded bike. I detail my first impressions with the Cannondale under load in Solo Without Pie: First Ride, in my sig line. I'm still very glad I went to an aluminum touring bike instead of sticking with a steel one.
Steel is about three times stiffer than aluminum. It's also around three times heavier. Those are about the only acceptable objective facts that can be said in this flame war that you're trying to start. When it comes to bike frames it comes more down to design than material choice. Steel frames can be made stiff and aluminum frames can be made noodly. With modern manufacturing processes and tubing choices manufacturers have a relatively easy job of choosing which type of bike frame they want to create.
It also needs to be kept in mind that the generalizations and wisdoms of ye olden days do not apply to modern frames since nowdays we have hydro forming, CNC's, oversized tubing and a myriad of other bike related standards / technologies to allow manufacturers to achieve either stiff or noodly frames. It stands however that noodly frames seem to be a thing of the past except with maybe smaller boutique manufacturers since all bicycle frames sold in the EU need to pass EU bicycle stiffness regulations, which are pretty strict. This applies globally since bike manufacturers are not likely going to start making different product lines for different continents just because they can vary stiffness levels. More likely is that they'll accept whatever regulations are in place and go with that.
Now as to not being able to pedal standing up, that can happen if the frame in question is in fact noodly. With any modern touring bike flexes or wobbles should not happen when doing that. It certainly does not happen with my 240lbs of body weight, 60lbs of gear and my puny 62cm steel LHT. It did happen with my late Trek FX 7.3 which was aluminum. It wasn't however a touring bike and wasn't designed to carry such loads comfortably.
As to OP's question, he did not want to buy a new bike, he wanted to know how to manage with his current one. Since it's a old MTB it's likely it's made of relatively low diameter steel tubing which can in fact be quite flexy and thus difficult to handle with heavy touring loads. If OP did want to buy a new frame, the material should be least of his concerns since every well designed modern touring bike will achieve sufficient stiffness to climb out of the saddle / throw the bike around even when heavily loaded regardless of frame material.