For those who've owned both, is there a noticeable difference?
The most major overall difference is probably weight. Alu watch is just
way way lighter, especially the non-LTE version, as the case materials are plastic, aluminium and glass, rather than ceramics, steel and sapphire, which are all denser (much denser in some cases) materials than their corresponding counterparts. LTE alu watch does inherit the ceramic back of the stainless model tho - not 100% sure why - probably because "classy premium material feel", I suspect.
Alu non-LTE watch with one of the lighter armbands is so light that you don't really notice it's there at all; you feel it clasping your wrist, but not its weight. So if weight is a concern for you or you're otherwise an active sportsman for example it might be worth considering the alu watch for this reason. Also, it's considerably cheaper too of course...
The glass display of alu watch is as mentioned more transparent, less reflective and tougher compared to the sapphire display glass of stainless watch, while the sapphire display of stainless watch is (way) harder. Just for example, my watch is an almost 2 years 9 months old first-generation OG sport model ("series 0" as called by some people); I thought I had just a few tiny scratches on it, but studying it recently under a single lightsource in an otherwise darkened room showed off a bunch more scratches that I never noticed previously and even a slight fuzzy hazing of a part of the surface (possibly caused by the hem of the sweaters I like to use rubbing right across the display hour after hour), which while not visible under normal circumstances is still there. The sapphire screen really wouldn't look like that; while sapphire is not impossible to mar (some people mistakenly believe you need something of equal or higher hardness), you'd have to work way more to scratch it so if you do scratch it at all it might just be one or a few small marks. Not the multitudes which is there on my display.
Alu watch case is also matte, due to the sandblasting and anodization process used to protect against surface oxidation, which will trigger some peoples' mismatched-case-and-armband-lugs OCD...
If you are sensitive to materials or finish mismatch (first-world problem for sure), you may find yourself limited to just a few armband styles like the rubber bands, woven nylon, sport loop or leather loop bands. *shrug*
Anodization actually turns the surface layer into a sort of ceramic-like material, it becomes much harder than the aluminium case itself, so thus gives some protection against scratches. Still, if you whack your alu watch too hard against something it might still scratch or even dent, and if you've picked one of the colored flavors like space grey or gold, the base metal might show through the anodized surface permanently which would probably be an irritating eyesore. Stainless steel does acquire micro-abrasions fairly easily, but is naturally way harder than alu, and especially when cold-forged like apple does, it is in a wholly different league. You probably won't be able to dent the steel watch! (Actually even sawing into it would be tough.
)
Finally, the sensation of "premium materials" might make some want to pick the heavier, more solid steel model. I've heard reports of the plastic back of most alu watches becoming fuzzy with abrasions over time, which I don't really understand how it might happen (not unless these wearers are lizard people and their scaly hides mess up the back of their watches...); still apparently is something that might occur. The zirconia/sapphire windows back of steel watch (and alu LTE) is unlikely to scratch in any way short of if perhaps you live and work inside a sandpit... Also, a few people apparently have allergic reactions to the plastic back, but this seems such an unlikely scenario I wouldn't worry at all.
Sorry for the wall of text here... TL: DR eh!
because it lacks the Sapphire coating that the sapphire display includes.
Minor nitpick... The sapphire display is a solid sheet, not a coat. If it was a sheet it would look funky at the edges of the display due to each layer having a different refractive index. Also, risk delamination/chipping if banged and so on.
