We had and used both the D2 (5U) and TD-6 on the farm in the San Joaquin Delta.  Crawlers were the tractor of choice back then because of our light peat soils and high water tables.
I preferred the TD-6's starting process over the D2... still remember pulling that D2 rope over and over in cold weather!  For those that don't know, the TD-6's engine was both a gasoline and a diesel engine.  One would start the engine on gasoline and after the engine warmed up you would push/pull levers and the engine would switch from gasoline to diesel. 
I disagree with a previous poster that the TD-6 was a smoother ride than the D2.  My experience with the opposite, and when given a choice, my men always chose the D2 when it was to be operated all day.  We used those tractors for pulling grain drills and single row asparagus discs in the spring when the asparagus beds were still wet.  I do agree that the TD-6 had inferior engineering (other than the starting process) compared to the D2.  I remember the TD-6 always required more maintenance/repair attention. 

I forgot that I owned a TD-6 gas/diesel dozer; Glad I have these late model glow plug tractors. My die hard IH buddy said the later gas/diesel tractors had improved head design for better coolant flow, lessening the chance of cracking a head. Seems like anytime we saw an IH for sale, the owner would have at least one spare head on hand for replacement.
I like the ride of these, as they have a 5 roller track frame; I have had D2s that rode extremely rough compared to others, so It may be because of worn cross shaft bushings etc.
Komatsu? I drove one in the rice fields for a thrifty farmer once. It was re-powered from the factory Cummins engine to a used one swapped out of an over the road truck; I noticed an out of place toggle switch and thought to myself, no it can't be...they still had a set of Jake brakes on the engine and it was operable! LOL. Nothing like cruising along on your D-65 Komatsu D6C wanna be clone I called it, and hitting the engine brakes at the end of the row. JM

Ha! Thats to funny!

engine brake on a dozer. yep there's a laugh. cummins were the first to have engine brakes. they also were the first with direct injection. there's a lot of tech on modern diesels pioneered by cummins

Juiceman,reminds me of elementary school when a kid would ask,how many sparkplugs does a diesel engine have.I would say how many cylinders,They would laugh and say diesels don't have sparkplugs.I was already driving early IH diesel tractors so I knew they did.It was hard to convince them.Most today still don't know that about early IH diesels

Since we're on the subject and have experts here, I've always wondered how the IH gas/diesel engines worked, as I have never run one. Did one start on gas with regular spark plugs, etc., then switch over to diesel once warm? How did that work? I would think with the high compression the gas operation would knock like crazy? What about the carb - did it switch the intake passage or just shut the gas off and have the air intake through a wide open carb? Just curious, as I always wondered how those things actually worked.

Here is a pretty good video

Gary, you remind me of my own smart**s hahaha. There was a guy on Fakebook arguing with us about how stupid some of us were, because he said an IH had a pony motor also, just because the tractor had a separate fuel tank. We are still waiting for his final response...
I am no expert on gas/diesel IH or anything for that matter; my limited experience with them (although I misplaced one that I still own):
Open gas valve; set gas/diesel lever in down position to change flapper/compression and diesel throttle off; start on gas whether you prefer cranking by hand or a push button if so equipped;choke is optional; let it warm up on gas for awhile, then open up the diesel throttle and quickly flip the gas/diesel lever up. After the diesel is running, turn gas off at tank.
It was recommended by many to idle down and switch back to gas mode to help it start easier the next time, which I never did.
That is why I like the 62 series, no gas to mess with, just push glow plug button in for several seconds and presto! JM

rhartwick - If you have the time to binge-watch a 59 episode YouTube video series, I highly recommend Pete's rebuild series of his Farmall MD tractor on his "Just a Few Acres Farm" channel.  Pete isn't a career mechanic but still does a good job of explaining the "how's and why's" of the tractors he restores - the engine rebuild of the MD shows exactly how everything in the IH system was built, how it operates, and everything that happens when the gasoline-to-diesel switchover occurs, all delivered in a way that even a layman can understand.  Here's the playlist where those videos can be found -  the COMPLETE guide to the Farmall MD, including how the starting system works! - YouTube

My compliments to everyone who has contributed to this thread, I love hearing about the performance and comparisons of the equipment from folks who actually used them in real world applications - the true proving grounds!