If you go with the heftier system, you'll never have to purchase a set again when you decide to get a heavier trailer someday. I had 14,000 / 1400 Equal-i-zer's but my trailer was heavier and my truck was a 1 ton dualy. Overkill? perhaps, but I never had issues of being under-par. Your choice, but if mine again, I'd go with the heftier system.
I gave my Equal-i-zer system to a colleague at work who purchased a Keystone Passport (approx 8000 pounds) towing with a 1500 Expedition. He said it towed with no problem at all. The WD system worked just fine.