Engines are almost always easier and more of a known quantity than anything else BUT the condition of the rest of the machine is more important.....especially undercarriage. On the surface it sounds like the 955 is your machine.....newer, track loader vs dozer, lots of spare parts sitting around.
I never worked on one (had a narrow escape involving a 933 once!) but undoubtedly a track loader is a lot harder to work on than a standard tractor. There are some press fit components on all Caterpillar equipment that you need porta-power equipment and tooling for, but other than that its just heavy iron.
Sometimes its just best to hire the pressing if you ever need it done. This would be things like sprockets, possibly steering clutch hubs etc. Not really familiar with the D4 size machines vs the smaller and larger tractors.