The faster top speed is due to the final gearing/ratio...4 mph is plenty for the vast majority of operators.
SmartDrive has been around a few years, and going through some running changes in design. most notably the elimination of the internal cone-type clutch, which reduces moving parts, weight, complexity, cost, etc. Rear drive speed on newer SmartDrive and SelectDrive models is done by progressive tightening of the belt between the crankshaft and input pulley of the transmission. When you move the control, you actually rotate (ever so slightly) the entire transmission case, causing the drive belt to tighten up and transfer more torque from the engine to the drive axle.
The hydrostatic design on the HRX has a constant-tension belt, and it operates a small hydraulic pump inside the transmission. Pushing the drive control level tips a swash plate that allows pressurized fluid to enter the hydraulic motor and turn the drive axle. The more the move the lever, the more pressure is delivered and the faster the wheels turn.
On the commercial model, the hydrostatic transmission is driven off a PTO shaft direct from the engine; this set-up is a bit more mechanically efficient than belt-style designs.
I've used all three, and while there's little doubt the shaft-type is the most robust, it's a very heavy mower. I prefer the control layout of the hydrostatic models but many others love the Smart/Select drive controls. Properly cared for, any will last for years and years. Keep the underside clean and do all the scheduled service and you'll only eve need to buy "one" Honda.