I'd go for the lightest one in this situation. Both are very strong and both have been on the market forever. The differance between ISIS and Octalink is that Octalink is Shimano's version of ISIS b/c they dont want to ingtegrate their stuff with other companys. I have a pair of 03' LX cranks and they definately are an improvement over my old Specialized Strongam II cranks.
Good Luck