What you gain
Weight: a 42mm titanium watch weighs ~80-100g vs ~140-180g for the steel equivalent. Wear it all day and the difference is real. Corrosion resistance: titanium doesn't rust or stain even with prolonged saltwater exposure; better than steel for serious dive use. Hypoallergenic: titanium doesn't contain nickel and doesn't trigger the contact-allergy reaction many people get from steel. Strength-to-weight ratio: titanium is stronger than steel by weight, though steel is stronger by volume.
What you give up
Hardness: pure titanium (Grade 2) is significantly softer than stainless steel; visible scratches accumulate faster. Grade 5 titanium (Ti-6Al-4V alloy) is harder and closer to steel scratch resistance. Polishing difficulty: titanium doesn't take a high-mirror polish as easily as steel; satisfaction in the polishing trade is lower. Cost: titanium case manufacturing is 20-40% more expensive than steel due to slower machining. Heat conduction: titanium feels warmer on the wrist than steel because it has lower thermal conductivity.
What modern brands use
IWC Pilot Titanium and Aquatimer Titanium use Grade 5. Tudor Pelagos uses Grade 2. Sinn uses both depending on model; the U-series military watches use submarine-grade titanium. Citizen Super Titanium uses Citizen-developed Duratect surface hardening. Grand Seiko uses 'Bright Titanium', a proprietary harder-finish grade with Zaratsu polishing. The premium tier (Patek, AP, Vacheron) generally avoids titanium because precious-metal cases are part of their value positioning.
When to choose titanium
For dive or active use: lighter on wrist, better corrosion resistance, more comfortable for long wear. The Tudor Pelagos and Omega Seamaster Planet Ocean Titanium are the canonical luxury divers. For nickel allergy: titanium is the obvious answer. Avoid titanium if you want a high-shine polished finish or if you're sensitive to scratches; steel keeps its polish better. See wiki: titanium grade 2 vs 5.