They're different sites, anyway. TF2WH is trading with bots, they also don't pay as much as the item sells for on their site. For example: Brotherhood of arms is selling for 20,000c, If you're willing to sell your Brotherhood of arms, they will buy for 18,600c. If that makes sense.
Outpost is trading with other users for items. Sometimes selling to Outpost is better, even though I haven't sold a thing. I'm new to the whole trade thing so please don't hate me ;c.
I admit, I prefer to buy things on the WH, but on Outpost I think it's better to sell.