PV = nRT
pressure x volume = moles of whatever x gas constant x temp
The ideal gas law.
Measuring the temperature and the pressure is redundant. they are directly proportional.
What machines are you looking at?
PIDs are useful on single or double boiler machines where the boilers only sustain one temperature of water at a time. you can precisely control water temperature with a PID.
Pressure controlled boilers are usually HX (heat exchange machines). Here, a PID is totally useless in a direct sense. You can measure it, but it will give you something like 250F or 280F... I have no clue. I'd be kinda interested to find that out, actually.
The point, though, is that it doesn't matter on an HX what the temperature in the boiler is. The brew temp could be anything for any different machine at a given boiler temperature, depending on the length of the heat exchanger. The thing with these machines is that the pressurestat is accurate enough. It doesn't make a huge, huge difference where it is, as long as it's in range (.9-1.2 bars). You control brew temperature on an HX primarily by a cooling flush... I guess it'd be nice for real geeks to have a little thermistor in the brew path with a highly accurate monitor on it, but I'd much rather play it by taste, personally, and save the extra money it would cost to have that added to the machine. Honestly, you can just let someone else do the dirty work of testing the flush lengths and measuring the avg. water temp at each volume, and go off that. It's what most people do, that or follow the "water dance" guidelines.
I think that making coffee is a lot more complicated than just reading a bunch of dials. Having more dials that give you quantitative feedback, from what I understand, don't do much more than give you more feedback.
Your best bet is to get either one (the only PID machine I know of is the Expobar Brewtus, which is not that great of a PID, I have heard, and there is another one similar to it that is coming out from Chris Coffee by Christmas, I think. Check the boards) and learn how to use it. No matter how many dials you get, you are probably going to make swill for the first while unless you're already a coffee king (like Chris Tacy, heh).
Get the best equipment you can afford, of course. The better your equipment, the easier it is to blame yourself for all your crappy coffee. Then you'll get a feel for your machine, and make beautiful coffee together with it, whatever it is.
Sorry if I am wacky/all over the place. just some disjointed advice.