I only have experience on a Garrett and it is a ACE 350 which I like. The 250 is very similar and for coin hunting would be great from what I read. To keep them interested I would plant a few coins discretely and watch their eyes light up when they discover them. This will more than likely hook them into a great hobby. I even get discouraged when swinging all day and no hits. But I keep the right frame of mind on it. Not every day will be a success. If I find about $1 in change in a hunt I am happy. Still looking for that valuable coin or at least a old coin that has real silver!!!!
Good luck and as far as the other brands mentioned, I am sure they are just as good. I do like the ACE readout, easy to understand and usually spot on in what you find.