The Subgear SG50 is a cosmetically altered and rebranded Mk 17 300.
Subgear is a Scubapro brand that was created to give Scubapro dealers a lower cost product line that could be used to compete with on line retailers. I don't think it worked the way Scubapro thought it would.
The Mk 17 is the best diaphragm first stage currently being made. Unfortunately, Scubapro is still very much a piston regulator company and it gets second billing to the less reliable in cold water Mk 25.
The C300 is an adjustable version of the C200, and as noted above they are unbalanced second stage designs intended for the European market. Divers in the US have a strong bias against un balanced second stages and they associate them with low performance designs. That's not the case with the C series regulators. What divers don't grasp is that with a balanced first stage, where the IP is constant, the forces in a second stage are always balanced anyway. Pneumatic balancing of the second stage actually makes a difference when it's attached to an un balanced first stage.
The C300 is designed for high flow rates and it wouldn't benefit from balancing on a balanced first stage anyway.
The C350 is an evolution of the C300 using a oval shaped poppet which eliminates some potential issues with sand retarding movement of the poppet in the grooves in the air barrel on the C200 and C300. My understanding is that Scubapro increased the tolerances on those models anyway to address the issue, but the new poppet and air barrel design is a more elegant solution.
Scubapro has a long history of supporting discontinued models, and with the parts commonality with the C350, support for the C200 and C300 isn't disappearing anytime soon. It might be an issue 20 years from now, but I doubt it as the O-rings are standard as it the poppet seat.
If you're worried about it, get a couple new poppets, a new air barrel and a new case (all of which would cost you less than $50) and you'll have all the parts unique to C series that are likely to ever fail. Or just buy a used on off E-bay for $75-$100 as a spare.
