I bought this model because the external model was out of stock. My Aprilia dealer charged my 30€ to install them.
The advantage with the internal model is that you can monitor the pressure while filling your tires. The disadvantage is that a wheel removal will be required when the batteries run out. It is not clear to me if you have to buy new units, send them to Chigee (the instructions mention "factory replacement"), or if they can be opened to change batteries.
What I did notice though is that the data listed on this page does not match up with the included instruction sheet. Here is the data from the instruction sheet:
Battery life: 2-3 years
Battery replacement: Factory replacement
Size: 25x25x49.5mm
Measurement accuracy: 0.01 bar
Range: 0-8 bar