A 6" rear 5 spoke is a tight fit on single sided monsters unless you run the spacer.
The 7 spoke is offset more so it will clear the brakes and curve of the swingarm that can limit the chain adjustment range on the single sided swingarm Monsters.
The 6" fit the double sided swingarm with no problems at all.
The 5 spokes are slightly lighter than the 7 spoke wheels- however the outer rim of the 7 spoke is lighter so you have less MOI. The 7 spoke should perform better.
The 7 spoke wheel is also considerably stronger. The 7 spoke design was developed out of the Supermoto wheels where they were tested successfully on 60ft jumps.
The additional spokes support the outer rim better.
-M