Depends. I wouldn't worry too much about their knowing how much you're getting for your house - they don't, after all, necessarily know how much other money you have or what your total funding is. The most important thing is to get an agent who's clued in to the market in your area, who'll advertise appropriately, and who won't turn buyers off.
I wouldn't even contemplate selling my house with the EA we're buying from, because said EA is such a colossal pain to deal with; I am selling with the crowd who sold it to us, because they know the area, have had several comparable houses on over the last while - so underbidders can be directed our way, which is actually how we bought the house in the first place, having been underbidders on another one. Not to mention that they treated us decently then, while doing their best for their clients.
Go with someone you feel comfortable with - if they make you uncomfortable, they make the buyer uncomfortable, and an uncomfortable buyer will give you less money - and who knows the area and the amenities. I made extra sure, and did up a crib-sheet for the agent: what the neighbours are like, public transport, heating costs, restaurants, schools, things like that.