Its part dependant on the property, if its in a good location, and priced right, a sole agent should be able to sell it reasonably quickly. You could neg a better fee too. Drawbacka are that you might have to sign with only them for a certain period of time.
You might get charged a bit more fees wise if you do a multi agency, but the chances of selling your property will increase.
If you need to choose an agent, choose one which is nearest the property you want to sell, an established local agent in the area is usually best, and instruct another name agency. Try not to sell with too many agents either, hope this helps