I do not own a Gaurdian but, I have heard that with the dropzone rest is activated it slams down into the recessed metal gaurd and hits the bottom of it making a clinking sound. The ripcord is essentually the same as the QAD Ultra Rest but just a bit cheaper (price and qaulity). On the ripcord I have been told that after a season or so the spring mechanism breaks. If it were between the three you have mentioned I would stay with the QAD.Also there is a review by Greg (one of the moderators) about a Muzzy Zero Effect Rest. It is worth reading, here it is:
Pass the salt... MAJOR kudos to the Muzzy Zero Effect Rest. Good Luck!