I want to use PostgreSQL for my standalone .NET Windows application. I downloaded PostgreSQL Advanced Server from EnterpriseDB's website and it is very smooth. The GUI also very good. But I suspect how along EnterpriseDB will support it.
The second option is to download from postgresql.org.
I want an edition that lasts long and I can easily upgrade it to future versions without any problems and it must work smoothly with .NET.
I use the version from postgresql.org, because this is the "open" version. EnterpriseDB has added features, which are open, too, has created an one-click-installer, and is just the "commercial" version of postgresql. BTW I assume, EnterpriseDB will support its version for a long time.
I however like the raw version to be sure to have the same functionality on all platforms.
You will always want the latest version (unless there are know issues), as for binaries so long as they are the latest version it shouldn't really matter - EnterpriseDB for example is just compilation of different parts which are free anyway (such as including pgAdmin) so it doesnt matter if they stop supporting it - you can always get support for the individual components - here is the list of them: http://www.postgresql.org/download/product-categories
I have been using 8.4 with .Net front ends with an uptime over a year. I mostly use Npgsql in .Net apps or you can go the ODBC route (which has the downside of requiring clients to configure their ODBC as opposed to just distributing dlls with your app - so I find is better suited to server apps) for which the offical driver is psqlODBC: http://www.postgresql.org/ftp/odbc/versions/
You can upgrade your postgres installation if it is within the same version eg: 9.1 --> 9.2 but you would require a new install for different versions, eg: 8.4 --> 9.1, but then you can easily dump your tables and restore them into the new install..