I use SIT strings C6 tuning on all my steels. I have never tried any other brand of strings. 3 steels are maple, 3 are 6061 aluminum. The wood steels stay almost in tune all year long.
By almost I mean very close to being spot on. The aluminum ones aren't bad, but you have to figure if you tuned them while they were on your lap and "warmed up" they will be one thing. If you pull one out of it's stand at room temperature, chances are it will be a little flat until it reaches the previous "warm up" temperature you tuned it at. 70* vs 98.6* (that's a big difference)
Most pedal steels use aluminum for the body. I figured there had to be some merit to this. It was the reason I built 3 of mine out of aluminum. There is a reason, pedal steels have a unique voice aside from the pedals and knee levers. I really think aluminum is superior to wood, for a lot of reasons. My aluminum ones are clad top and bottom with 3mm acrylic, which I think helps to aid the keeping the temperature more stable. (insulation) So, I guess I think maybe the material your steel is made from has more to do with strings staying in tune?

Currently own, 6 Groner-tone lap steels, 1953 Alamo Lap steel, (a gift from the late Stu Schulman) Recording King Phil Leadbetter Dobro, Roland Cube, Roland Mobile Cube, Fender Champion 40