I've got a 300 size reel I'm about to spool with some bulk mono I have laying around, I've got CXX in either 20lb or 30lb that I'm trying to go through, but can't make up my mind which to use.  The reel will be paired with a 8' Heavy rod designed for 2-7oz baits, but will stay on the lower end of that rating, 2-4oz swimbaits.

20lb

I have not fished that line.

 However according to the manufacturer,  the 20lb. is listed at 0.017 in and the 30lb comes in at 0.022 in.

 Seems both tests have a slightly smaller/thinner diameter than the industry standard.

If I could still make the casts I needed to and get bites, I'd go for the 30lb; especially since it's only 0.002 inches larger than most brands 20lb.

btw - I like 25lb.

Good Luck

A-Jay