well I use a good flat iron that works amazing for straightening and curling my hair, it's the Karmin G3 Salon Pro Styling Iron which doesn't damage the hair and keeps it soft, shiny and smooth. if you are interested you can check it out at safrons.com :P
As such both will give you pretty spectacular results , do be careful to use a heat protect though. If you do have a little time though i would recommend not using either as both cause hair to become drier, thinner, splitter and frizzier .... foam curlers on towel dried and combed hair , along with some hair product to suit your hair type is the best bet
Honestly, I have curly hair myself, and I wanted to get it straightened but it probably wouldn't stay too long. I wouldn't use a curling iron, it can damage your hair, but if you don't have curly hair you might use it sometimes. I just use gels and stuff to make the curls stick. This can work if you have wavy hair or even straight hair but it depends on how you apply the gel.