I am not a materials expert, but my basic understanding is: titanium alloy is stronger than steel; pure titanium probably is not as strong as steel; titanium alloy is a lot harder to machine because it is so tough (and that is why it is more expensive); pure titanium is a lot easier to work with but is not used very much; and, you probably do not have to worry about excessive wear of titanium cogs unless you start to see sparks flying from your freewheel.