The trencher suggestions are good ones, if you know you only have a few, or that they will always be narrow. The backhoe lets you dig BIG holes and big trenches too.
If you get a QA bucket and front remotes, you can buy or rent a trencher attachment for the front of your tractor, assuming you get a tractor with enough HP to drive it. This is where the skidsteers excel.
For narrow, shallow trenches, say for a water line, a cheap middle buster can be used on a tractor. Very fast. And there are some other inexpensive implements specially designed just for this purpose too.
If you go with backhoe, here are few things to consider:
Weight of tractor is king - will help you get the most power out of your backhoe
Reach is a consideration, limited by tractor size: 20 hp = ~6.5', 25-35 hp = ~7.5', 35 - 45 hp = ~9', etc. Don't under estimate it.
Get subframe mount only, NO 3PH backhoes.
Good dirt and/or pavement pads are a must.
Good large (1/2" hoses) to rear remotes are better (faster). Rear remotes make backhoe on/off a little easier than long plumbing. Then rear remotes can be used for other purposes too.
Pay attention to hydraulic pump capacity of tractor. More = faster working.
Get a proven backhoe from a reliable manufacturer.
Consider a true TLB (like a Kubota B21 or B39), flip over seat, closer hoe to tractor. Heavier duty.
For CUTs, I think Branson is the heaviest, followed by Kioti and Mahindra. Kioti's have excellent pump capacity. The Kioti CK20 is good for it's size, if you don't need the reach or more hp power for other things. But, I'd opt for the bigger CK27 (or CK25). Or, if you think you need the PTO HP, the CK30 or CK35. I also like the TYM T273 or the new T353.
I'd definite check them all out, and look used first.