Watch-case titanium is one of the lightest and most-corrosion-resistant practical metals; both grades are ~40% lighter than 316L stainless steel and entirely hypoallergenic (no nickel content, no skin-reaction issues). The difference between Grade 2 and Grade 5 is the alloying content: Grade 2 is essentially pure titanium; Grade 5 is titanium alloyed with aluminium and vanadium for substantially higher strength and hardness.
Grade 2 (commercially pure titanium, CP Ti): ~99.2% titanium with trace iron, oxygen, and carbon. Vickers hardness ~150-200 (softer than 316L stainless steel's ~200-250). Easy to machine; brushed Grade 2 has a soft matte grey appearance. Vintage and entry-tier titanium watches almost universally use Grade 2: vintage IWC Titanium references, Seiko titanium pieces, microbrand titanium. The trade-off: scratches easily from normal-wear contact; the soft matt finish becomes visibly worn over years.
"Pure titanium scratches like aluminium. Grade 5 scratches like steel. Both look the same on the shelf. Buy Grade 5."- Watch retailer on titanium grade selection
Grade 5 (Ti-6Al-4V): titanium alloyed with 6% aluminium and 4% vanadium. Vickers hardness ~350, harder than stainless steel and approaching tegimented hardened steel. The alloy is the standard aerospace structural titanium (used in jet engines, surgical implants, and racing-bike frames). In watchmaking it is significantly more expensive to machine (the harder alloy wears tooling faster) but produces a case that is highly scratch-resistant; the brushed finish stays visually intact for years.
Industry adoption: IWC Big Pilot Titanium and Pilot Titanium references use Grade 5; Omega Seamaster Planet Ocean Titanium and selected Speedmaster X-33 references use Grade 2 or Grade 5 depending on year (modern: Grade 5); Citizen Super-Titanium uses Grade 5 with a proprietary surface hardening (Duratect) that further increases hardness; Grand Seiko uses Grade 2 in vintage 1970s references and Grade 5 in modern.
For buyers, the practical guide: Grade 5 is the better material for daily wear; the additional CHF 100-200 cost differential is worth it for scratch resistance. Grade 2 is acceptable for occasional-wear pieces or where matt finish is desired. Both grades are identically hypoallergenic and lightweight; the difference is purely surface hardness. Some brands market "titanium" without specifying grade; this typically means Grade 2 (the cheaper option).