Carrierwave or Dragonfly

Question

I have been looking into rails file upload tools and the ones that seemed the most appealing and interesting to me were carrierwave and dragonfly.

From looking around it seems like carrierwave takes the more traditional style where you can process the file on save whereas dragonfly is middleware so it allows you to process on the fly.

I was wondering if people had any references to performance test or any test that compare the two.

Also, just curious on what people's opinions are about both and which they prefer and of course why they prefer it.