-----

#1 I think if you are doing a GTA type of game, all of the for mentioned items should be dts as they can be animated, e.g. breaking fences, knocking over mailboxes, or things that can be picked up.
DIF's are used mainly for "static" objects that cannot be moved/damaged. I use difs for all buildings and dts for anything that can be moved.
There are many posts related to this and has been discussed many times. I will post a link to one as soon as I find one.

Edit: Did a search for dts dif and came up with this link that sums it up:

www.garagegames.com/index.php?sec=mg&mod=resource&page=view&qid=7459

#2 Main differences, from what i gather anyway, is collision calculation. But here's something that might help.

Think of it this way: Anything that a character can potentially walk on should mostly be a DIF. Anything that a character can potentially manipulate (i.e with hands) should be a DTS. So, Houses: DIF. Mailboxes: DTS. Stop Signs: probably a DTS. As for a fence, that's kind of an "iffy" one; i would say DIF right off the bat. There are of course reasons why you might use one over the other in certain situtations, but that's getting complicated and i won't be the best at explaining those reasons.

#3 ----

#4 Well Static DTS's can cast shadows, it requires some code change, haven't botherd to do that yet. DTS can't cast shadow onto other DTS. I'm planning on doing all DTS objects since i hate them silly map making programs so much.