I own a 2004 Volusia. The Marauder 800 has an older engine design, and is a cheaper bike (it is made to be their entry-level bike). However, since they are close in terms of price and design, it is really up to you to choose the one you like best. The Volusia is very easy to drive, looks like a much bigger bike, and gets consistently excellent reviews from press and owners alike.
Check out this
review of both.
This is a comparison of the Intruder 800 and Marauder 800.
The 2004 Kawasaki Mean Streak and the
2004 VZ1600 Marauder are the same bike, if you are referring to the VZ800's larger cousin.