The country you are in will make a difference as the device selection does vary, but it sounds like you are using devices for the US market.
In any case, just be aware that in general Zigbee motion sensors are a bit quicker than the Z wave ones, but not so much that it matters to most people.
Quicker Response Typically Equals Shorter Battery Life
There are devices which have adjustable parameters and you can make them more sensitive, but be aware that that will reduce the battery life, sometimes significantly. Normally manufacturers will ship a Device intending for it to get about 12 months a battery life. If you change the parameters to make it more sensitive, you will reduce that battery life, sometimes down to two or three months.
Next, before we discuss specific brands, make sure that you have the Sensor placed for maximum efficiency. Many people think that the typical PIR motion sensors are like a camera and you should place them so they are facing straight on towards the oncoming person. But in fact they are measuring very small changes in heat passing across the detection field. So you want to place them so that the oncoming person will be walking 90° to the sensor. This can make a difference of a second or more on many sensors. See the following FAQ:
FAQ: Where to locate motion sensor for fastest response?
OK, onto to specific brands.
Fibaro offers the most parameter settings, so you can get it to probably be the quickest, but at the cost of battery life as discussed above. And it does tend to be one of the more expensive sensors. Note that there are two identical looking models, one for HomeKit and one for Z wave. You need the one for Z wave, the other one does not work with smartthings.
I will leave others to discuss other individual brands, there are quite a few to choose from.
Zwave Direct Association
Your idea of using Z wave direct associations would be a good one if there were an easy way to set associations in smartthings. Unfortunately, there isn’t. It can be done with custom code, but most of the manufacturer provided DTHs don’t include this method because they assume you’ll be able to do it from the hub UI.
There are a few battery powered devices that can be set in this way using the Z wave tweaker, although the code author says it only works with mains power devices. It just depends how the device handles accepting configuration.
(As @Awestun has pointed out in another thread, fortunately the fibaro multisensor is one of the ones that you can use the tweaker with, because it has a configuration button on the device itself that will allow it to accept the changes. But some other battery powered devices will only accept configuration changes when they are first added to the network.)
If you want to talk more about that option, just let us know. Note that in order to make zwave direct association work, you are going to save the configuration in the trigger device, in this case the sensor, not the target device, which in this case is the switch. So you want to see if the sensor has parameters that allow you to set associations.
Also know that if you use zwave direct association you no longer have any control over that rule from the hub. It will happen very quick, but it will happen every single time. You won’t even be able to say you only want the light to turn on at night. It will be that every single time the sensor is triggered, the light comes on. So that’s another factor to consider.