DVIP: Closer to the original game, no (or very few) additional event or graphics mods included, improved cultural setup that builds on the original one.
TASS: Broader scope than the DVIP and thus very different from CK-DV "vanilla", event mods included, includes graphics mods, has a very modified cultural setup, has two additional scenarios.
Personally I have both, in two different CK-DV installs.
DVIP is very much a team based effort in that I have based much of the changes on community suggestions. If you plan on picking and chosing your own event or graphics mods, DVIP is easier to do. DVIP games can be converted to EU2 or EU3 with only minor issues.
TASS is a much more complete mod and more a total conversion in parts. You will find a graphical mod, many included events, and a very changed cultural, duchy, and kingdom setup. As a result applying other mods to TASS can take some work to make them compatible. TASS games cannot be converted to EU2 or EU3 since the converters are based on CK-DV unmodded.