i am thinking of adding a caterpillar  to my farming tractors.   what are the plus minus opinions between the D4d SA and the D5 SA?  Thanks

I'm sure that you will get more technical answers from others, but my comments come from owning and using D4s, D5, and D6s that I used in farming.
The D5 was a very good tillage tractor in its day.  The functional difference between the D5 and the D6D was not a big as the functional difference between the D4D and the D5.  I am referring to the SA models that had standard shifting transmissions commonly used in farming.  In other words, the D5 could handle any of the implements that I had as well as the D6.  The same was not true with the D4D.

How are you going to compare the D4 4-cylinder with the D5 with 6-cylinder?

thanks Delta that is interesting. i didn't know the D5 would weigh in that good against the D6d.

You could always own one of each! Seriously, the size of the implements might be a factor? Not too big/not too small?
What would be the tractor be used for mainly? High speed tillage, or deep tillage? Dozer work, or just playing around?
I am a very fortunate guy, as I have a really nice low hour D4D SA that is a turbocharged 3304. It has an amazing amount of power, and is pretty compact.
Moving up, I also have a D5 SA with a 3306 naturally aspirated. It is plentiful for disking, etc., the power steering makes it nice too. However, for ripping, the previous owner did not care for the gearing in it, claims it was too fast. His solution was going with a D6D SA VHP machine that had double reduction gears.
I had a nice D6C 74A here, some renters did not like that model, saying it was too slow. Their preference was a D6C SA 17R, but known to self destruct the finals, etc. from high speeds.
So many choices! JM

used mainly for disking and later pull disk with a couple of drills attached behind the disk. flat sandy loam with close water table. wheel tractors will work but once over and tracks up bretty bad and gets boggy. what size offset disk will a D4D of D5 pull. for reference i can pull a 14 foot offset or tandem disk and a 20-8 grain drill with my D46U same conditions.

another question. i have no experience with the SA cats. i see on some tractors an extra gauge mounted either in the dash or on top that looks to be three or four inches in diameter and usually is red green yellow with a pointer. i assume this is either a tach or a boost gauge? something to to with variable horsepower?

Sounds like a late D4D SA or even a D4E is plenty for you. The D4D I mention has a LOT of power. I have pulled as much as a JD 18’ semi stubble disc with ring roller with hardly any effort. The JD I prefer here in the west are offset model 1200 series, which would pull harder than your tandem.
The gauge is a speedometer/tachometer. It shows speed/RPMs and “ suggests” up shifting if RPMs are high.
VHP machines from what I have encountered, will have a red dash light when in the top gears. Mine on the D4E has been set up to run wide open in all gears.
Do you have a specific tractor near you available for purchase, or shopping? 
Can’t wait to see whatever you purchase. JM

i was considering purchasing a disk that would fit a D5 maybe so i could get more done. same for the D4d. the only equipment i have right now fit the old D46U pretty well but i would like to size up some. right now just shopping and trying to learn the ins and outs of AG CAT's. from my limited experience with the D46U i wonder why i haven't done this many years ago. all of my land lays together with only 1 mile between two different properties so getting moved shouldn't be an issue.