I have to agree with FAT FAB. Welding aluminum on a steel top leaves stray arcs on the work due to poor grounding. An aluminum top doesn't mark the work or cause arcing. For SS, the Aluminum top acts as a good heat sink to control distortion. A steel top allows you to weld fixtures to it for forming, or jigs, or just tack welding the piece directly to the top. I've used both tops, but for primarily doing either types of materials. A good compromise to this dilemma would be to build the table out of steel, and have half the top steel, and the other half aluminum. When I set up my home shop I will build a table half and half, getting the bennefits of both worlds. The cheapest way to buy aluminum is to find a shop building things with primarily aluminum, and buy a piece of crop from them. Those shops usually will sell a piece of crop for scrap value, usually about 1/10 the cost of buying through a regular supplier.
Indeed. A hybrid design would give the durabilty of steel and the advantage of aluminum, allowing the best of both worlds. GREAT idea!