Well perhaps if you back up a little and try to explain what your are trying solve or achieve rather then various methods one might use.
I have a device that will have a constant power supply on the pcb board (it is an AC-to-DC power supply), and will provide the option to debug/service the device in the field without turning off or removing the power supply ( it will be hardwired in). I just need to make sure its OK, or rather make sure I include a method to plug in the usb on the fly without damaging my computer or the board itself. Not sure if I need a surge protector circuit just in case plugging in the USB creates some kind of back surge into the USB for that split second before the switch (I think Im rambling here
Frankly I've always felt the arduino auto-voltage selector circuit was quite a waste of board space and not as fool proof or flexible as it should be.
How flexible should it be?
I very much liked the method the very first model arduino boards used, which was a simple 3 pin male header where you placed a jumper chip on one side or the other to choose USB or on-board regulator to power the board. The Seeeduino 328p boards also have a nice manual small slide switch to choose the power source. The space taken up and the cost of the extra components could have been better spent on a small RTC chip or some other useful and common function.
Board space isnt exactly a problem with me, as the parts are extremely small, and I am quite creative with running traces. In fact, I would rather tiny SMD parts as opposed to a large, bulky 3-pin header that takes up a lot of space. Besides, I already have an RTC on board, so that isnt an issue.
In the meantime, I am using the approach that the Arduino Micro uses (shown below), which got rid of the comparator all together, and just uses two p-channel mosfets to switch between usb and vin power sources. It seems like a much more efficient way of determining which source to use, without the use of the comparator chip. I also am using a tiny part (sot23-6 footprint) which caught my attention, which uses two reverse diodes to protect the digital lines on the usb port (not sure how necessary this is).