The BEST advice I can give is to visit the forums on both sites and read the posts there as well as here on those smokers. A large forum with lots of members speaks volumes. SI has a large following here on SMF, you cannot miss it. There are also some posts regarding the ST line.
I will also say that you need to read the FAQ on the SI and ST sites. The temperatures do have swings, and they are normal for an analog smoker. For the vast majority of users, those swings are accepted. For some however, tighter control is desired and they build a PID or add one like the Auber and get a more precise control over the temp, usually within a couple of degrees. Nothing made by man is perfect.
I would also look at the warranty, and the cost of accessories or replacement parts. Also compare them feature by feature be it wheel size or capacity, wattage, whatever.
You can join the forums and ask unit specific questions there as well. Shipping is by FedEx at SI, and it is not cheap. But it is quick. I don't know how ST ships.
You cannot go wrong with either smoker. The users of each will tell you that.
If you have deep pockets, you might want to look at the CookShack line of smokers.