I have stayed at the Hilton twice and it is an okay hotel - relatively expensive for what you get but of course you are paying for the location! I stay in hotels quite often for work and I would not rate it as any better than okay.
Personally, I will try Le Merdien (Sofitel now) next time so I can compare, assuming the prices are similar. If you book early you can get a great price. We have also stayed a couple of times at hotels not at the airport and to be honest, the money we saved on the hotel cost was almost wiped out by the cost of taxis to and fro the terminal, and the more pricey hotel meal you end up having by default. At least when you are onsite at Gatwick you have an abundance of relatively priced meal options in Gatwick Village. I would just clarify this by saying we almost never have our car with us, it might be worth staying offsite if we did.
So my vote is for the Sofitel, although I have not actually stayed there.