Hi or Low depends on what you are trying to get off the vehicle. A low PH presoak should get inorganic (road grime) off of the vehicle. The Hi PH presoak should get organics off the vehicle (bugs, bird droppings, and greases).
Monte, it's more of a question of what you are trying to get off the vehicle than which one you should use.
Presoaks in an IBA are hard too. Most presoaks need a dwell time to work. Most IBA it's hard to get a dwell time because what are you going to do, just stop the machine and let it site for 15 seconds? No, that's not going to work.
Ask and check with any chemical manufacturer you use what the dwell time should be and what the recommended dilution rate is. A lot of companies like to put "up to" rates that may or may not work. If you can get the company to give you three dilution rates, an up to, a recommended, and an economy use, that would help a lot.
Here's the hard part; you will probably have to change the tips in your machine to change the dilution rates to match the new chemicals. Very few are all the same. It's something that not every car washer owner learns, but it is something you should know and understand.
If you need any more help send me a message and I'd be happy to help.