Presenting a eulogy for a loved one could be the saddest thing and the most difficult public speaking endeavor you could ever experience. The level of emotion and grief in conjunction with the enormous details upon which to be decided can result in even greater turmoil.
Just sit down and write from your heart. Services are really about love. The most recommended eulogy is a combination of both since death should not be a reason to be sad but a celebration of a life well lived.
This is all very hard to do in the little amount of time you are actually giving the speech. The service belongs to the family and is conducted as they wish. Some people actually find it easier to stick to a theme, during the writing process, making it a bit easier for the words to flow.
If you are gifted with the blessing and talent of writing poetry for the dead, share it. It is not something that can be written quickly.