A few things about aluminum pots:
1) Remember that before you use it, you need to build up an oxidation layer, or else aluminum will leech into your brew. To build up an oxidation layer, just boil a pot full of water for about 15-20 minutes, then dump the water. A little discoloration from this oxidation is normal.
2) Oxygen-based and caustic cleaners, like PBW or Oxiclean, will actually eat through the oxidation layer, so you don't want to use these types of cleaners on your aluminum pot, unless you want to re-establish the oxidation layer after each cleaning. Stick to acid-based cleaners, and never scrub the walls with anything coarse, like steel wool, just use a sponge or something soft like that.
3) Aluminum conducts heat better than stainless steel. This is both good and bad. The aluminum pot will heat up water more quickly than SS, but it also will dissipate heat faster than SS, so it will lose heat as fast as it transfers it. One of the positives to this is that you will also be able to cool down in a ice bath faster than you would be able to with a SS pot.
So that's pretty much the pros and cons. If you are cool with all these points, go for it! I personally have both an aluminum pot and a SS pot, and I perfer the SS. Go for the aluminum if it is strictly a financial decision to get started on homebrewing, but if you can afford it, save up for a good SS pot, which you should be able to find for about $60-80.
Good luck!