I will assume by your question that you are not represented by a Realtor, otherwise you'd be turning to him or her for advice.
So - it's difficult to give you negotiating tips in just a few sentences! The best I can offer is to call the 1st buyer (btw - why do you prefer him to the 2nd, higher offer??) and let him know you received a higher offer - let him know you'd like to stay with him, so want to know if he will reconsider, and then offer his "highest and best" number.
If the terms of the first offer are that much better (ie - all cash, or a high down payment, or, the buyers have nothing to sell, etc) then you might want to make sure you do nothing to scare him away!