Hi,
Have you seen Scampys recent thread on pretty much the same topic?
In my experience using "normal" PWM (normal as in hundred to thousands of Hz since that what the PIC usually generates using HPWM etc) doesn't work with SSR for controlling AC and I try to explain why in the other thread. However a hot tub is a relatively slow system (if you don't have a heater in the megawatt range) and you can easily do a software PWM with a period of something like 10 seconds (or even longer). Do a timer interrupt at 100Hz, keep track of 1000 interrupts and you have your PWM signal with almost 10bits of resolution.

What other consideration should I think about?
Safety safety and of course safety!