My mind always goes to resale/trade in value since these devices will eventually be slow or unsupported. Apple Watch has the worst trade in value of any Apple product so spending you money on anything more than a base model aluminum watch is literally throwing money down the drain. If you plan on keeping the watch until the wheels fall then see below:
Alum - lighter, body is less prone to fingerprints/scratches, screen is more susceptible to scratches so if you're someone who bumps into things best to avoid.
SS - heavier, body is more prone to fingerprints/scratches, screen is more scratch resistant but not true sapphire like "normal" watches so proceed with caution.
Titanium - middle ground between the other materials as far as weight with the SS attributes.
Remember these are tech watches so spending hundreds or thousands with their atrocious resell values and short lifespans is not the best financial decision.