The standard 18000 spool will definately do the job, if you want another spool the power drag spool would probably be the go as it has a steeper curve, you only want the 20000 max if capacity is an issue or if your fishing pe 12 and want to fit more on
The most popular SOM no limits spool is the 16000 because it beefs up a 10000sw into a reel capable of stopping large GTs, it has more drag, a steeper curve and greater line capacity (you can fish heavier pe) than the standard spools. I have not played with an SOM 18000 so i cant really comment but i would say its probably not required on an 18000sw