There is a thread about the x650, find it and you will have plenty of info.
S600, awesome reg, pretty much sets the standard out there
MK25 first stage, delivers lots of air with no problems, also an awesome first stage
As far as the MK25SA, its a waste of money, as is the titanium version.
Think about it, you spend an extra $150-600 for a lighter weight first stage (the standard isn't that heavy anyhow) and then have to pack it right back on with extra lead. I say save the money and subtract a few ounces of lead from your setup. Don't get me wrong, they are also great pieces of equipment, but they don't work any better and they are a hell of a lot more money.
MHO on the X650, this regulator has gone through revision and revision and recall etc. It isn't as common as the S600 and will likely be replaced with another version before the S600 does. Still a great piece of equipment, but most shops I have seen sell many times more S600 setups than the X650 setups. My guess would be that these shops are likely going to have parts on hand, and if you have a problem on a trip, you are much more likely to have a resort shop carry parts for the S-600.
Scubapro, stands behind their product, is large enough to not be gone tomorrow and backs their product up with awesome warranties and the lifetime free replacement part program for your regular maintenence.
the Classic, we don't have that one in the "States"