There are pros and cons to using both stainless and aluminum. Here is a link to one such discussion.
I am assuming you will extract brewing, so in that case either will be fine for your five gallon batches (size wise that is). It will really all come down to personal choice and how much you are willing to spend. In my opinion I would go with the stainless as it is only a $25 difference. Just remember that if you do end up getting the aluminum that you should boil some water in it before you use it for brewing to build up a passive oxide layer in the pot (I have also heard you can just throw it in the oven to do the same thing). Here is a good podcast on the Brewing Network about metals. Listen to this as it may give some more insight on how metals react with brewing (http://thebrewingnetwork.com/shows/B...fect-Your-Beer
Good luck and welcome to HBT!