Depends on your ignition. IF you still use points, the 5ohm coils are best. You CAN use 3 ohm coils, but it will chew through your points and condensers much faster.
If you use electronic, you CAN use 5 ohm coils but your spark energy is less, though the spark is supposed to last slightly longer. The 3 or lower ohm coils with have a stronger spark to them. Some ignitions require lower ohm coils for best operation.
I have a dyna S and the 5 ohm coils are perfectly fine for that system.
The ride IS the adventure. The destination is just to get gas!