I have owned two Aluminum versions of which the cases are in pristine condition. There aren't any issues with case durability. I've used a film screen protector on both and so the screens are in perfect condition, just as when I received them. I don't use a case for my watches, use them when doing yard work, anything outside, hiking, biking, fishing, tennis. Anything. If I were working on a car or doing mechanical type of work I'd probably consider a case or removing the watch.
There are aspects of both that I like. When I'm exercising, it's hard to beat the light weight of the Aluminum. As I've said, I don't have a single mark on the cases, the screens have been protected (go with ArmorSuit, nothing has been better).
SS is just over the top beautiful, doesn't matter which finish, they all look amazing. It's noticeably heavier but not uncomfortably so than the Aluminum. The screen on the SS is slightly more clear than the Aluminum but both are comparable. I also think it dresses up a little better but as I've said countless times, with Smart Watches, and Apple Watch specifically, the stigmas of traditional watches go out the window. Everyone knows you have an Apple watch and it doesn't matter what you wear it with, which band you wear it with.
Right now the SS is the better value in my opinion. 47% off the list price is an incredible deal.
Edit: One thing to add. With traditional watches, I know that Sapphire Crystals are more resistant to scratches but quartz crystals are more resistant to impact. In theory I think that's what Apple was trying to get at by going with the different crystals of the Aluminum vs SS models. This article seems to support that.
Which one is the better glass for displays in smartphones? We are testing it, ion x glass vs sapphire! Let's compare and find out.
www.glaz-screen-protector.com
There are many here who have had the stainless model for quite a long time and swear by them. Same can be said for Aluminum. I'm one who can attest to the durability of aluminum first hand having owned them the longest. I also feel like people dismiss the quality and durability of aluminum models more than is really the truth. Honestly, I think if you take care of your watch and use common sense, you'll be fine with either choice.