Depends.
Fox is a proven name with proven reliability. I'd also trust rockshox though. It basically boils down to personal preference.
Some people will buy a Nissan Sentra and be perfectly happy. After all, they are decent, reliable, functional cars that get good mileage and are extremely reliable. Some people wouldnt be caught dead in a plain old Nissan and have to spring for the Infiniti because they think people will assume they have lots of money if they drive an Infiniti, regardless of the fact that it's just a rebadged Nissan...let's just call this the "bling factor".
I know plenty of people like that...and am friends with none of them. Just because something costs more, doesnt always mean its better. In fact, sometimes, you are LITERALLY paying for the name and are getting an inferior product.
In THIS case, however, I honestly think the two are pretty equal. The price difference is because FOX probably feels it has the reputation advantage because most pro-level bikes are supplied with Fox hardware....although its probably only because of the bulk rate discount they give to manufacturers.
I know it's not the same thing, but I like to relate stuff like this to things other than bikes. Take Glock pistols for example. They like to claim that their guns are used by more police departments than any other maker, and that fact somehow means their guns are better. The FACT is that the only reason more PD's use Glock is because they basically gave them the guns for FREE if they were willing to trade in their old issued firearms. So lets see here: keep our old worn-out guns or get some brand new ones that arent falling apart for FREE...gee..whatever shall I do? See what I mean? There are plenty of guns out there that are both cheaper AND better than Glock...but because of their marketing practices, everyone automatically assumes that Glocks are the best when in fact they are not.
Same thing with Fox vs. RockShox.