'evening Smoque,
I believe you have made a good choice on the SI3. Plenty large enough for just about anything you care to smoke. Regarding the PID. It is not necessary to provide good bbq. It will provide temperatures with less swing (see the FAQ section). Temperature swings are normal, and over the period of time necessary to do ribs, or a butt, or what ever you cook, it will average out to the temperature you set. This is common for all analog smokers, regardless of make. And the food is really good.
The PID reduces greatly the temperature swing to just a couple of degrees + or - of your set temp. For those who demand that type accuracy (or reassurance that all is well), it is a good investment. And you can set them to cook to a predetermined internal temp like Wik stated and that is a nice feature.
And as PolishQ said, $ for $, you come out ahead with the SI3 and the Auber and cart when you compare to the competition. Just be sure you are looking at the same capacity for the smoker, or as close as possible. You really cannot go wrong. Good luck.