I also have an older JBJ ATO with float switches ($80 give or take without the pump), got it used, never used it myself, got kind of nervous with the float switches as I was starting up my tank this year because of invert shenanigans, they really do get into everything so unless your running a sump and you can isolate it in the return chamber it wouldn't be my first choice.
But if budget inst going to allow for a 130-200 dollar unit with optical sensors, then find a unit you like that has decent reviews that you can afford, just follow
@radiata 's advice and snail proof it.
When I have to snail/invert proof something I use this...
http://www.homedepot.com/p/Frost-King-E-O-6-in-x-20-ft-Plastic-Gutter-Guard-VX620/202518614
or similar, very cheap and works wonders, easy to work with, scissors and Zip Ties. I have used it on my DT overflow, my sump (prevent larger inverts from going back to the media chamber), and used it to keep my sand sifting start fish from knocking over some frag plugs in the sand bed on occasion.
Also, another word of advice...when you hook it up, only keep enough water in the reservoir for a day or two until you know it is running smoothly and have a good handle on how much the system is going to need on a daily basis and gauge your reservoir size on how long you want to go without refilling that. It's kinda scary when you come home to an alarm and your recently filled reservoir is empty, happened on my coral QT, optical sensor was getting too much light and when the sensor tripped, it stayed on. The same can happen with a float switch with a snail or invert sitting on it or wedged between the glass and the float. The Coral QT is a 20 gallon fusion and I had 2 gallons of RO water in the reservoir. Water didn't overflow, but the tank was filled to the brim...salinity was down from 1.26 to 1.12...for a week after I only kept half a gallon in the reservoir until I figured out the light was causing the issue. Luckily it was my Coral QT and it was just cycling, no livestock! I only use float switches in my RODI storage container everything else is optical.