There are far too many unknown variables than merely 5mm poplar plywood vs "sheet aluminum", and to put it concisely, their different properties have different failure modes, reshaping potential, and fastening requirements.
I find it hard to believe that you are in an engineering class but did not see the significance of mentioning some particulars about the design as well as type of aluminum and gauge or thickness.
What is this "sheet of aluminum", exactly? Even if you used a metal brake to bend it "some", it would be difficult to get more than one or two bends when you could be aiming for something tubular (hollow) for high strength to weight ratio.
I take that back, you could construct a sheet metal shelf and then it depends on your resources to bend it, but this is a poor strength to weight and cost ratio, but perhaps the easiest way to DIY get the job done. In that case it depends quite a lot on how many bends you put in it, which depends quite a lot on what the metal brake is capable of since you don't have a way to die press them to an optimal shape... OR perhaps you do! A shop hydraulic press, and a die made out of hardwood, could be sufficient if there is significant surface area to work with, to make more intricate shapes than a metal brake alone could do, or I may be barking up the wrong tree if you already assumed you'd be welding separate pieces together.
Since this suggests that the problem is manufacturing rather than potential material strength, I'd shoot for about 2mm thick aluminum in a fairly effective design and thicker 3mm+ for something where you can't fasten or bend a more complex shape. Keep in mind that you can use as thick as your metal brake can bend, then drill it out to reduce weight later. This will be an expensive loss of material, but in a real manufacturing environment, the scrap metal has value rather than going into a landfill.
Hollow round or oval would be best for that but introduces additional challenges in connecting the pieces unless you are adept at welding aluminum, well enough that the welds do not become the weakest part of the design, and they may still be and reach the target goal but it's something to consider.
Square or rectangular tubing will be easier to connect using fasteners and is a bit more forgiving than wood at suffering shock damage, but will also eventually suffer fatigue from it so it is something to consider, that it not just support 50lbs but for how long in a real world use.
I could go on and on but ringtail has it right, that you need to do some testing of the specific material you intend to use, then change materials or design if inadequate. Change materials doesn't mean don't use aluminum, but rather stating aluminum is like stating wood, a material alone does not tell much of the story.
Backing up a bit, apples to apples, if you have 5mm thick plate aluminum in exact same design as wood (except the fastening method may differ), it should have no problem supporting 50lbs, is quite overkill in weight and expense rather than using much thinner tubular structural members as mentioned above, or a framework of solid core round pieces if you can weld it. The size that the platform needs to be, may determine whether solid core or hollow has the most merits.
What if you just keep your wood prototype as a core, then slap some fiberglass on it?