If I couldn't find a suitable contractor and was tasked with digging my own ¼ acre pond, I'd probably rent a dozer, or a trackhoe, depending on the design of the pond, and the placement of the excavated material. A dozer will move an equal amount of dirt over a distance far quicker than a trackhoe will.
If for some reason I couldn't find a suitable rental, and I just had to buy a used machine to build a ¼ acre pond, I'd probably choose a backhoe over a tracked machine. Of course, it'd take longer to dig the pond, and I'd want to carefully plan my attack in regards to proceeding in and out with the machine, and the possibility of having to drain rainwater. But, not having to worry about the time constraints of a rental would allow me to work at my leisure, and I'm all about leisure.
A backhoe is far more versatile than a tracked machine, and can be used for numerous other tasks on the property. Backhoes are less expensive to haul, maintain, and repair, plus if it does break down, chances are, I can actually tow it back to the house to work on it. And when I'm finished with the backhoe, it'll likely be much easier to sell than would a tracked machine.
Disclaimer: The above is merely my opinion, which has been formed after witnessing, and hearing other horror stories of the 'joys' of bulldozer ownership. lol
