Recently I researched buying a new kettle, but did not pull the trigger. (Bought a new guitar instead. Choices... choices...) Like you I currently brew 5 gallons but would like the flexibility to do 10, so when I do buy that next kettle it will be 15 gallons.
One issue with size I found was that some of the preassembled 15 gallon kettles, like the Blichmann model, put the thermometer at around the 6 gallon mark, making it somewhat less useful for 5 gallon batches.
I also wholly recommend getting or modifying a kettle to have a ball valve. No reason that you need to be lifting or even siphoning.
On aluminum vs stainless steel, it's your call. Aluminum is cheaper, but requires more maintenance and isn't as easy to modify (ie., drilling ports and/or welding; refer to Palmer's How To Brew). I had an 8 gallon aluminum pot for a while, I stopped using it because I upgraded to full boils and had problems with boilover, and also I managed to pit the interior by leaving some cleaning chemical (or possibly temp storage of Star San) in it for too long. Those sort of problems are less likely with stainless.