From:
http://www.new-spa.us/faqmicroneedle
Stainless steel needles are made of medical grade SUS-304 steel (the same grade of steel used for manufacturing of hypodermic needles). The thickness of the wire, used for the needles is 0.20mm in diameter. Titanium needles are made of medical grade SUS-304 steel covered with titanium vapor - the composition widely used in implant surgery. It makes the steel stronger but at the same time the wire becomes thicker up to 0.30mm. The punctures made by titanium needles are larger and more visible.
The span life of titanium needles is 6 month VS 3 months for stainless steel.