neither. I've had lots of root canals but never had the chance to go skydiving; I'd take it if it came along. Being kidnapped by aliens with anal probes ... that worries me a bit.
I have a almost lifelong morbid fear of dentists and a fear of heights gained much later in life. Of course i would rather do neither but if i absolutely had to do one or the other, i honestly believe i would choose to go skydiving.
I once worked with a girl who had that fear. When we had a fire alarm (and use of lift was prohibited) I had to carry her down the stairs (one flight only fortunately).