Well since you mention it... my machine has just started cutting a bed out of 25.4mm aluminium plate, £50 worth of which will be turned into swarf after 20 hours machining! It's essentially a T-slotted bed with an array of roughly 300 M8 holes for additional mounting options.

It all depends on what you want to cut? In my opinion aluminium profile should be avoided since a rigid bed made from this would be exceptionally expensive. If you want to machine aluminium regularly, then a bed made from strips of aluminium or a single plate is far superior. If aluminium is only occasionally going to be machined, then MDF is fine but for long runs it's not good due to it absorbing coolant.