The Arduino solution will have less components (simpler circuit - less things that go wrong) and you can easily change the "logic" by rewriting the program.
My personal favorite for the water level would be the same arrangement as in a toilet - an arm with a float - and a simple potentiometer. (as always . water and electroincs do not mix - you need to think twice & thrice about keeping the water away from wires) It might be a softish plastic rod and use a force/bend sensor rather than the rotary motion of a potetionemeter (No moving parts - more reliable)
The connection to the house alarm : is it just an external circuit switch you open/close (which the house system sees as "Water Problem!") or does the Arduino have to do complex communication (WiFI/BlueTooth...)? In the simple case an Uno should be ample.
Someone in the General Electronics forum may have an opinion how to run a signal for 15m - usually this is a current loop if sending an analog value, and any old twisted pair if sending a serial/digital information.