I like a monitor, as the ph reading is readily available. I can see the ph anytime i walk by my tank, without having to do anything.
I got a used laboratory grade monitor/controller for $25 and a new probe for $60. Works well and from across the room in large bright red LED numbers I can instantly see my tank is 8.19!
The probes don't really need replaced that often. They don't fail(please verify this, i am not an expert, but my probe is 2 years old and still works well), they just take longer to get a reading as they age. For a monitor, where the probe sits in the water the whole time, a quick reading isn't neccesary