If money is no issue, I would recommend a Cookshack as it is digitally controlled. I have yet to have an issue with my unit and the temp is pretty much dead on, fluctuating up/down only a degree or two. The "computer board" that regulates the cooking temp clicks incessantly and early on I thought it was dying. The folks at CS said it was normal.
The ST and SI units are similar, analog units. My brother has the model 3 from SI and it is a solid unit, works well and turns out great food. I have no experience at all with the units from ST. Both provide some feedback via a light that is on when heating and off when not heating. There are temperature swings with these units that can vary from a few degrees from the set point to 10 or more degrees above (heating) to below (not heating). This is normal. With these, you will want to get a good digital thermometer when smoking a large cut to an internal temperature. A dual probe thermometer will also let you monitor the units internal temperature (and worry and agonize over the difference between what you set and what the thermometer is telling you).
The angled control panel on the ST unit is more attractive than the flat top on the SI. The SI units have a longer cord and larger wheels, and costs a bit less.
Another benefit for the larger units is a dual door latch. None of these units (CS< ST< SI) use door gaskets. It is a compression fit. Two latches better distribute the pressure for a more complete seal. Also look at the wattage of the heating element within a given dollar amount. All the units have an element properly sized for the capacity they are rated at. Additionally, check the price of accessories/repair parts from each company. You will notice the accessories look identical, yet the cost can vary. And then look at the warranty in detail.
Finally, read the forums on the manufacturers sites, and ask questions of the owners.
You will not go wrong with any of the units you are interested in. They are all built well, and the companies will make everything right should something not work properly.They all have a very strong following. Sometime this year, SI will be introducing a unit or units with a digital control per their forum.
CS will periodically have a scratch n dent or remanufactured unit for sale. That is how I was able to afford the 066 model. It never hurts to ask.
Good luck.