Pretty basic. Figure out the cost to rent, thats your direct discount from buying. You will need other projects to "justify" the cost of ownership. When it comes to excavators folks alway say "I want one" so if you can affird the money out, then get it. I like to buy used stuff at market value, then sell for the same as I paid, its like borrowing it.
In my case I bought my KX41 after the economy rebounded, had to pay $14k + $1k repairs, so I may be a little underwater but at least having fun & not renting, transporting, or waiting. KX 41 is a tiny machine, but it doesnt take up space in the barn and I can tow it with my F150 and without worry, lots of good aspects that make up for a smaller bucket.
One huge benefit of ownership is the project can be "done" on your own schedule. A few hours here & there. And you can change your plan anytime. Some projects are better like that. To rent you must have your ducks in a row & slam it out. Shape of the pond, where the spoils go, weather on your side, etc.
Lots of replies on TBN recommend to slam it out, don't consider savoring a project. Which is understandable its hard to advise someone how to screw around, even if thats what he's doing in the overall picture. Members picture themselves doing that job for someone and want to appear (in their mind) professional, not goofing off, so the Replies tend towards "how to be productive".
Sometimes you dont want to hurry, what the heck are you gonna do AFTER that? I like to slam out the ones I don't enjoy though. I do things both ways and each can be extremely satisfying.,,,, and have no problem finding more projects (too many actually).