Don't dismiss that it is a hard choice. At the beginning it seemed daunting. However, after going thru the batches you will start to find yourself being swayed by the same type. Also something to take in to consideration is the different avenues of where you are finding your donors. Some agencies are more expensive then others. This might help you decide with who you want to work with.
Some people it is height related. There are many folks who will also classify donors depending on what type of under gradute facility attended. Also depending on where you are cycling, some of their requirements might cause you to look at one donor over another. One local clinic near me would not allow someone over the age of twenty nine to be a donor another would not let someone who carries the cystic fibrosis gene.
Good luck in your search.