My guess is that grade 2 would be significantly cheaper to mill and polish the case, with little difference in the amount of wear a watch would get.
Grade 5 is not "better" but is an alloy with al and vandium. It is a little harder, and therefore harder to work with. Grade 2 is "pure" Ti and is used in alot of medical grade stuff, and 5 is not.
Some reading on Ti grades.
http://www.supraalloys.com/titanium-grades.php
http://www.mrtitanium.com/tigrades.htmlClick to expand...