As far as I know, the 2328 is the "Professional" series, and the 2128 is the "Professional II" series, which is actually the upgrade. Have a look at http://www.eheimparts.com/client/pro...x?categoryid=4
The way they split the availability of their model numbers between the American and European markets is very confusing. For example, the "2328 Professional" does seem to still be available, but only in Europe. Their top of the line "wet dry" filter, the 2329, is also only available in Europe. In America you can only get the 2229, which is the same thing but doesn't have the integrated heating.
The other thing that I think stinks is if you want to buy the 2128 for your Marine system, you have to pay for the filter which only supports fresh water, then you have to pay $432.38
at eheimparts.com for the case and heater that supports saltwater if you didn't think to look for a vendor that sells it as a "saltwater" version.