muttley,
A dozer is unlike other equipment that has front and rear tires. When you cross the center mass point on a dozer going forward, it quickly falls forward and down. In muck, especially deep muck, you can very quickly get the front-end buried when it falls over that mass point into soft deep muck. The only time I've been hurt on a dozer was crossing that mass point and being thrown forward into the top of the dozer...a nasty gash in the head was the result. Seat belts are absolutely required.
Slopes can also be tricky. Even though the center-of-gravity is very low on dozers, they can still roll over. When shaving the embankment on a steep slope, you have to really watch it. I'm basically a coward on a dozer and when it feels wrong I stop and try to attack it a different way....or hire a professional operator
. However, even if it rolls, as long as you are strapped in everything should be fine.
Both of those models you mentioned should work just fine. I have a Case 450 which is comparable to the D4 I think and the maneuverability of that size machine in a pond is really nice.
As long as you are alert to danger and always place safety ahead of everything else, you should be fine....one nice thing, you can always use the D7 to get the D4 unstuck.
