I mostly agree with mr_phillip. Neither watch is an investment, but the SS is nicer in the traditional way. A stainless steel watch looks more professional and most bands out there have SS clasps which I think look odd with the Aluminum watches.
That being said, I bought an Aluminum watch (Nike+) and I will most likely get an Aluminum watch next time. I like gadgets and I will most likely update it within a year or two. The SS is overpriced (some of the $1,000 SS link Series 0 were selling for $300 brand new when the Series 2 was released). Several years ago I owned a lovely stainless steel watch with a stainless steel link bracelet that I think cost under $100. The metal really shouldn't make that big of a price increase, but they make loads of money doing it, so I can't blame them. (Same with the gold edition. There were companies that would put your Apple Watch in a solid gold housing for $3,000 where Apple charged $10,000). Anywho, if I were to get a SS Apple Watch it would be for the look, so I would probably get a Series 0 and mainly just use it to tell time. I don't really have any interest in this though since I think the Space Gray Aluminum looks great and I won't feel bad upgrading next year.
I'd hate to buy a SS version now, and then want the new version next year but guilt myself out of it because I just wasted so much money this year on the SS version.
Of course, we all spend our disposable income on different things, and we all have different amounts of disposable income, so my opinion isn't worth much, but my vote is for the Aluminum.