Ken, thanks for the inquiry. First off in reply, comparing the SLC5/03 to an S7 really is apples and oranges, in fact its like comparing a shrivelled dried crab apple to a nice fat juicy navel orange.
The PID instruction in the SLC-500 is a pretty basic integer based (no floating point) instruction with only minimal alarming and no bells and whistles whatsoever. By comparisson the Modicon 984 PID uses just 21 words.
Compare that to the enhanced PID instruction in the Control Logix, which is probably closer in memory size to the one in your S7 (The CLX also has multiple variations of PID instructions)
I've successfully done temperature control with the SLCs pid instruciton without modification, but in order to do pressure control I had to add a lot of extra code. Any PV, ROC, CV limiting, bumbless transfer, cascading, manual control of CV, I-term manipulation, etc, all require lots of extra coding on the SLC.
I suspect that ChrisFlint's application will not need a lot of fancy PID enhancements and the SLC will handle it just fine.
The SLC also counts memory in 16 bit words, while the S7 is a 32 bit PLC. So when comparing the SLC to a 32 bit machine, multiply the size by 4. IOW, a SLC style low end PID control block on a 32 bit machine would take up 96 bytes.