I have the aluminum and the SS. Regardless of which you prefer the looks of, I'd get SS purely for the durability. I'm still using my 1st gen SS watch and it still looks great, and I can polish it at any time to make it look practically brand new. I've had my aluminum one for only 4 months, even with a case and screen protector it has a lot of little nicks in it. And when you get a nick in the metal it looks white, so against the black body, they are very noticeable. If I didn't have a screen protector the screen would be scratched by now too. The aluminum is VERY delicate.
I'll be getting a gen 4 in SS. Trying to decide between the black and silver. I had the silver before, and always thought it looked tacky. I want the black, but the only downside is it can't be polished.
Yes SS is more expensive, especially now that they only offer the cellular option. But if it doesn't break the bank for you I think it's really worth it. It's going to be the one thing you look at every day and that's attached to your body more than anything else. And with the little that Apple updates it, you probably won't need a new one for like 3 years.