I have done both and I would choose the Devils Den.
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
With all the talk of the swim throughs at devils den, i will add this warning.
There are false swim throughs!!!! If you are not careful you can find yourself in a dead end with no way to turn around. There is one in particular that i got into which was a dead end and when i finally turned to get out i found me self in another dead end. I kept my cool and found my way back out, but if could have turn out very badly.
Devils Den is actually a more advanced dive than blue grotto because of the cave openings and the false swim throughs. Be careful if you go there without good lights and cavern experience.
Tom.