How are the Pro's Choice SMB boots compared to similar brands?
I personally only like Pro Choice or Classic Equine Legacy. The Pro Choice Elites and the Classic Equine's are fairly similar. The Pro Choice don't breath as much but they are a little sturdier built IMO. The CE are nice because they breathe but they sacrafice the thickness and sturdiness.
What is the benefit (if any) of booting all four legs instead of just the front?
If you aren't going to be doing strenuous work, booting the front two is fine. If you are going to be doing things that are more difficult and strenuous I would boot all four. Really depends on what you're going to be doing and how much support you're going.
What activities/types of riding are the SMB best for(great for some, useless for others)?
I use mine for medium/heavy work outs, showing, trail riding...
What is the difference between the SMB II, the SMB III, and the SMB Elite?
The II's are the most basic model. They aren't very fitted and they are a little harder to find the perfect place for them to fit on the leg. The fetlock strap is a little sturdier than the Elites IMO. The III's are like a melding of the II's and the Elites. They are cut more like the Elites but have the same fetlock strap as the II's. They are also made out of the same material as the Elites (breathes more than the materials used in the II's). The Elites are the most lightweight and breathable. They are also a little bit more fitted than the III's and the fetlock strap is different. If you are buying a new set of boots I would go with the III's or the Elites.