Never used a Garret. However I have used a Minelab 705 on a rather trashy beach up here in NE Scotland and it does give off a lot of false signals. You do have a beach mode on the 705 but I find you do have to ground balance a bit more. Having said that, I have found quite a few old coins on this beach I visit with the 705. I am seriously thinking of using it again as I use a Whites Dual Field PI Machine and its starting to do my back in! The 705 also takes a while to pinpoint also. I suppose it depends on the beach you are on.
We aren't really supposed to compare different makes on these forums but there is a huge amount written about the capabilities of all the various individual detectors.