My current setup is a 25L kettle with a ball valve and thermometer with the BIAB method, it serves me well but I now feel another vessel would make my life easier and have been researching the pros and cons to both.
For me the main advantages of an all in one seems to be the obvious far smaller foot print and room taken up, generally cheaper, you have everything you need including a pump, software/timer and in some cases a chiller, there are plenty to choose from and they appear to be improved with now models so the quality is very good.
The plus points to going to 3V is that the setup may be more durable with thicker stainless steel and better components such as the pump and the use of tri clamps. You can replace/upgrade each vessel as and when is needed rather than having to replace the whole unit with an all in one. I suspect if you had an issue with a vessel you could just go down to 2 if need be to get you through a brew day, where as if the pump failed on a Grain father which has no tap during a brew you could be in trouble.