Take the offer that meets your needs the best and is most certain to close. Sometimes a seller needs a quick closing, the offer that meets that, even if its lower may be the best. You don't have to take it though, a great agent would tell the buyer's agent, "we have a higher offer but we like your closing date better, if you can improve your offer we may take it" or words to that effect.
A great agent will review the offers with the seller and discuss not only the price and closing date, but strength of the pre-approval letter, secondary details like inspection and other contingency time frames, the reputation of the buyerâ€™s agent (Some agents are miserable to deal with and should be avoided) and the entire package.
There can always be issues that are hard to calculate that may come into the picture as well. I present a cover letter with my offer explaining my buyers strengths and interest and their appreciation of the house and the neighborhood. I am always careful not to violate any fair housing guidelines, but I give the seller a reason to like my buyer even if they aren't the highest.