A basic router question

I have voip telephone service and have been making adjustments in my TPLink routers QOS section.

I have noticed that different routers use different ways to implement QOS, My old Linksys used a method where you gave a device high, med, or low priority. My TP LINK uses a QOS method where you actually assign a minimum and maximum bandwidth to a device.

It appears that with the priority method other devices can use the full bandwidth of the system if the high priority device is not in use.

But with the bandwidth method, a given bandwidth is set aside for a device and if that device is not in use the full bandwidth of the system is NOT available for other devices. I'm pretty sure of this because if I allocate a given bandwidth for my ooma devices and then run a speedtest in my browser, the speeds shown are significantly lower !!

Am I missing something or is the priority system for QOS a better system ???

Thanks for any help in advance.