I have a piezo element that I got from a song card, it looks like the one at: https://www.sparkfun.com/products/10293, and I was wondering whether it was safe to connect to my raspberry pi without a resistor? When I do it works as a speaker (buzzer) output but I am wondering whether a piezo buzzer would be better.
Yes it is safe to connect without a resistor. As for which is better a piezo disc or buzzer. There is not much difference (one may be louder than the other for the same voltage, and the may differ in tone), but only experimenting will tell you if there is enough difference to care.