bikes differ- swingers are adjustable,different length swingarms ect...
what bike?
just get a 120 link chain -( grind a link plate off )--( easy),and take out the links you dont need- (or buy a chain breaker)
O-rings typically last a lot longer, but a non-O-ring chain can have a greater tensile strength. There is more, but I don't want to start a debate on accuracy today, maybe tomorrow.
Take a look at this chart, and compare the numbers of similarly sized chains to help you decide what is best for you: http://www.didchain.com/specs.htm
i always just buy the longest chain i can so that i have nothing to risk. you can always cut it shorter. its better then buying too short of a chain and then having to send it back and get another.
go to the bike shop and take a look in the back of the Parts Unlimited catalog it will tell you your stock gearing and how many links the chain is and if you want a higher or lower sprocket just look at what it says in the book and add or subtract the no. of links from the chain