The DBC and DBU are the most closely matched products, both are basecoats made for exterior usage. DBC is the newer of the two and within time(its happening now) DBU will be phased out.
DBU will hardnen once you add in the DTR reducer, DBC will not hardnen.

The DBI and the other one you listed are basically made for interior usage. Not something you want for the outside. They were made for painting interior plastic parts, but the vinyl dye system (SX) works far superior.
In alreality there is no point for the DBI.

If you were going with PPG basecoat DBC is the route to go.

Currently working on custom metal and How to videos.

1949 and 1972 chevy trucks
70 chevelle SS455
Pictures of my work and projects
http://community.webshots.com/user/sevt_chevelle