My experience has been that dreamhorse.com is the site to find "deals" (lower priced horses). It seems to have a lot more "backyard" horses, which is great for buyers looking for deals but not so great for sellers looking to sell a top-quality mount. It seems to be more trail/backyard horses than show horses.
Equine.com is great, but they cater more to the higher-end show horses (not so many deals). You see a lot of dressage, eventing, jumpers, plus a fair amount of western show horses. Their $100 photo/video ads are great because they refund the entire thing if the horse doesn't sell.
So, I would say it depends on the market you are looking to sell to. Look for a site that has a lot of horses already listed similar to yours, because that's where the buyers are going to be looking. A buyer looking for a top-of-the-line eventer isn't going to waste their time shuffling through a thousand $500 backyard pony ads.
Do NOT use equine hits. Not only do they have very little traffic (and no ad response) but when I tried placing some ads there a while ago, they overcharged me. I contacted them at least 10 times through email, phone, etc and never got a single reply (and of course no refund). But I did start getting a LOT of spam (20 times more than normal) right around the same time I placed the ad! Scam, scam.