What's the difference between the 15 lb and 17 lb radiator cap?

[Bad89coupe]

I just was ordering a radiator cap from the local chevy dealer for my 89, and he asked me if it was a 15 lb or 17 lb cap? I didn't know since the car wasn't with me, whay the different pressures? This stems from my low coolant ligtht problem, I was going to get the cap and a thermostat to flush and clean the radiator and have it powerwashed. Now I am wondering what the two pounds of pressure difference would be for the car?

 

[silver 80]

check to see if you have the standard radiator or the heavy duty.
In my 87 I have the V01 option which is the heavy duty. and it takes the 17 lbs cap.