Two of these three decisions can be heavily debated. Well, PIC fanboys might debate my AVR decision as well, but for me the decision was easy.
Do I have a PICKIT? No.
Have I programmed a PIC before? No.
Do I have a USBtinyASP? Yes.
Have I programmed an AVR? Yes.
So with that an AVR is chosen. Next logical question should be the language to use on it. I would like to learn C well enough to use on mcu's, but for now, the one I feel most comfortable with is the Arduino Language. Sorry haters. If you do recreate my project, feel free to use whatever mcu and language you want. If you want to port it to better C code for the AVR I'm using, feel free to push it to my Github repo for others to use.
Then lastly, the cheap 315MHz one-way communication radio I'll be using. Yes, 40% of the reason I'm using it is because it is cheap, 50% of the reason is I have a pair in my drawer and 10% because I have used it before. I will however replace it later on with something better to have two-way comms and possibly a mesh network should you want to monitor a hot-tub as well, but for now, a virtual wire link over 315MHz will do nicely. I have received pretty good distances with these units as well (around 45m) so it will be more than enough to mount the base station inside the living room (only a window and 10-20m between PoolButler and base station).
So that's the reason behind my decisions. Love it, hate it, get over it.