Andrew,
Here is some info on the regulators.
First Stage The MK 16 (Yoke or DIN 200BAR/ 3000PSI Available) has the same high airflow Balanced Diaphragm mechanism found in the SCUBAPRO MK 18, in a smaller, more economical package. It features SCUBAPRO’s patented TIS System.
Balanced Diaphragm Delivers air in a consistent manner, regardless of tank pressure. PASS (Piston Assisted Seating System) Provides better airflow, even in high demand situations. Available in 3000 PSI Yoke or DIN System High Flow Low Pressure Ports Along with two standard low-pressure ports, the MK 16 has two high flow low pressure ports for better air delivery. Thermal Insulating System (TIS) Patented anti-freeze system. S550 Second Stage The S550 delivers many of the same performance characteristics as the remarkable S600, with the exception of diver-adjustable inhalation effort control. Inhalation technology in this premier regulator is factory set to provide fine-tuned performance with every breath. Main components are made from a special technopolymer, making it one of the lightest SCUBAPRO regulator second stages available. This advanced material also helps prevent freezing, eliminate corrosion and provide consistent air delivery at any depth. 

S550
Second Stage 
Compact Size Diver-Adjustable VIVA (Venturi-Initiated Vacuum Assist) Diver can adjust for optimum airflow during dive. Technopolymer Body and Internal Components Help prevent icing, eliminate corrosion and reduce weight. High-Flow Exhaust Valve For easy exhalation and clearing. Limited Lifetime Warranty Your regulator for life! Limited Lifetime Warranty Your regulator for life!
R390 Second Stage
The R390 regulator's classic design and downstream valve second stages are ideal for sport divers and diving programs. The downstream valve technology used in the R390 has been perfected by SCUBAPRO and has proven itself to be reliable and convenient to use. Another feature of the R390 is the user-controlled Venturi Initiated Vacuum Assistant (VIVA). The VIVA system reduces the amount of effort divers expend when breathing; this patented technology is user-managed and is available on all current SCUBAPRO second stages. The R390 combines a time-tested, fourth-generation downstream design with rugged construction and dependability.
Scubapro "300 bar" Din Conversion Kit for MK16, MK18, MK20 & MK25
I hope this helps,
Thank you,
Mike