I was about 30lbs heavier than OP for years. My experience was that aluminum was more reliable than steel. But the material is one of the least important considerations - first is the fit of the bike, then the suitability of the bike for the riding you intend to do.
When you have worked your way down the list of considerations towards the bottom, where 'material choice' is located, remember that the actual material is less important that the way the material is used. A superlight steel Italian racing bike from the early 80s will likely not be a good choice for a big rider, but a modern Surly-style (overbuilt) could be very good.