I find aluminum gouges easily, so I'm biased, but obviously I'd say stainless.
But give the Ultra a shot too. My wrists aren't the greatest, 160 to 165mm depending on who's measuring and when, but I was surprised; the Ultra didn't look like a behemoth after all and coming from Garmin -- reluctantly, due to the better health push notifications and Garmin's retreat from LTE support -- the battery life was the clincher. (But if you're smaller than 160mm the Ultra just isn't going to fit all that well).
The strap eyelet bands on the Ultra (Trail, Alpine) fit my wrist better than the elastomer ones (e.g. Sport) on the Series 7/8 because they fit the sides of my wrist more snugly, though the best case scenario for fit for me probably would have been a strap eyelet band (e.g. Milanese) on the Series 8.