AIO = All in One.. are mostly sealed.
You dont do any maintaince... the coolant is premixed with etyl to last long.. the tubing is plastic so you dont have evap happening inside your closed loop.
The only real mantaince you will need to do is keep your radiator clean from dust.. and dust off your motherboard collecting.
Odds have it your pump will die faster then you will need to refill the coolant.
I dont fully trust those pumps for long term usage...
Your betting the entire safety of your CPU on a made in china pump/motor which is less then half the size of what normal watercooling people use.
Comparing the crap AIO pump to a:
MCP-355 class or the MCP-655 class...
Its pretty simular to comparing a backyard electric kiddy car... to a tesla.
vs
Those 2 pumps also cost as much as your entire AIO's so yeah... you pay for quality.
personally id go air unless u dont have clearance on the motherboard and u need the slim fitting of the AIO.
You probably wont get any better temps then a a good air sink anyhow on such a weak system.
Also you will get more air though your motherboard and other IPC's so its a overall win vs a AIO.
That is my personal feelings about AIO.
I wont use them unless im forced to use them. (space reasons)
Air is a much better.. less headache.. less stuff to worry about route with near simular performance at the end of the day.