Like with a well chosen greetings card, the words are most important. These verses are hopefully saying something about your intentions and thoughts on the day. Presuming that your guests know the tune, meaningful text is surely the next priority. Unfamiliar words can be fitted to a familiar tune, but not vice versa.

Having cleared both of the above considerations, it might be worth looking at the vocal range required in your selection. Tunes requiring a wide stretch from low to high are best left until later in the Service: like any singers, your guests will gradually warm up as they sing more items. They need encouraging at the start.