First: I too use a pressure pro.. I will try to cover the basics of many systems, in many cases more than one system will fit the description but I may pick an example:
Smart Tire: This system is quite possibly the best but... It reports the temperature and pressure of the tire/rim (temp is rim temp) to the monitor, alarms on high pressure, low pressure and high temp. The "BUT" part is that you have to dismount the tire from the rim to install it, and it won't fit on towed's with "low-profile" tires. You also have to dismount the tire to change the battery in the transmitter. This unit is closest to the factory original system on many cars.
Most other brands I know of the sender mounts on the valve stem, in place of the cap. Thus it can be self-installed (DIY) and it is easily removed to replace batteries or sensors, you can also transfer from vehicle to vehicle. There are so many brands I won't bother with an example save for these
Pressure Pro older units reported the tire pressure of each tire to the monitor, you could check your pressure, at 55mph, while driving, by pushing a button.. It don't get much better than that, they alarmed at a low pressure, I think 12.5% less than when the sensor was attached.. That means if you attached in August, in Feb they were alarming. (At least in Michigan) I have had 'em alarm in the middle of the night due to cooling of the tire.
Newer ones you may be able to adjust the alarm points (nice) and they now have models that alarm on high pressure and send (And alarm on high) temp as well, Just like Smart Tire.
Some other systems just alarm on pre-set pressures,, IE: 100 PSI (or 90 or 80) and do not send pressure reports at all. Just "LOW/FLAT" and "Normal" (no signal)
Some do not tell you which tire is alarming
Decide on the features you want.. And then visit the company web pages to get the proper unit
I'd like to see some "improvements' in the system I have
I would like to see user replacable batteries (Pressure pro does not have that)
I'd like a smaller 2nd receiver for my towed (just 4,5,6 or 7 tires please (the odd one is the spare)
I would like a temp-compensated low pressure alarm so that Aug/Jan issue don't beep me
Home is where I park it!