Thank you to all for your answer!
Well, let's go to organize the ideas (for me!)
1) Raspberry PI 2/3: 40/50€ value on the net. Waste money to "simply" acts a relay, no?
2) Arduino UNO: 20/30€ for acts a relay. Yes, very very simple to control an Arduino wireless from a PI, all right?
So, my new project it can be following (let me know your point of view!)
A) Raspberry PI as LAMP server (with the web-based software on it) ==> Just done. I'm a PHP developer and I have written yes a full functional APP web-based that monitor temperatures, switch relays, log temps etc. connected with Raspberry GPIO.
Advantage: good velocity of execution as webserver.
Disadvantage: mSD crash etc. So the PI need really to be accessible in every moment, and not permanent fixed on the wall / on the heater. In every moment I want attach to monitor and check what's the problem / change SD etc etc etc.
B) ARDUINO UNO (or do you want tell me another model) permament fixed to the heather // wall with simply acts a relay and wireless linked to the PI (NRFL2401+). I think that there are so many sketch around, all right?
C) 1/2/3/n wireless sensor, programmed with Arduino, acted by Attiny85 + CR2032 + NRFL2401, to send temp and humidity to the PI. So many sketches around the net, I saw.
What do you think?