Ok most any detector should be fine away from salt, though you may be giving up alot not going to wet sand and wading for that matter.
Ace 400 is a ring/coin vacuum, very simple to operate, once you get to know it tells you more than you think between VDI and sound - Numerous times I have called out rings before I scoop them, after dozens of non ring hits. Does lack on depth, you can expect an alert down to about 7" or so on a nice fat ring, thats about it. Lack of depth never bothered me at the beach, not looking to dig trenches anyhow. AT is a nice machine, is all I use in the water, I wouldn't pay the extra premium if you have no plans to submerge.
Simplex is nice, bit more of a learning curve than the 400, bit more depth, lots of extra features most of which you won't be using. What I don't like about it, and most modern detectors including the Vanquish and Nox, is sub-par recovery speed, which means you have to swing reaaaalllll slow or you'll miss targets, a big handicap at the beach. Some might tell you ok tweak this setting and adjust that, turn this one up, switch modes and notch that one down and you have good recovery speed, just lose a little depth in return ! May be true but I don't like jumping through hoops to make something function, with the 400 recovery speed not a factor, it alerts every time without fuss and without compromise, which actually is what I like about it the most.
Things I don't like about it, no backlight, sound volume not adjustable.
