Hi, I'd say buy the RS one , even at 51pound, or 51/.65 (I think)= $78 AUS is pretty good.
You are also detecting low voltage as well not just phase failure, a low phase will make a mess of an induction motor without any obvious symptoms until it too late.
Phase sequence monitoring is also important if motor direction is important.
If you have them continually failing then you need to get the failed ones to an industrial electronics service workshop

, to find out why.
I have had a number of 3phase monitors die, usually after they are 3yo, the problem is dried out electrolytic capacitors, and failed X2 MKT type caps.
One particular brand has a comms bus in it so some major electronics, the 5V supply is full of 6V electrolytics, they all leaked or suddenly GREW UP.(This customer has 139 of them in use, and keeps 50 in stock)
Tom.....
