[Shred]

Where should be the "best" antenna, on the RX or the TX unit ?

I guess it depends on your definition of "best". I'd put the best aerial on the Tx, because a poorly matched aerial results in a significant portion of the transmitter power being reflected back from the aerial to the transmitter. If both antennas have a good impedance match, but are of different designs, then it really doesn't matter which way around they are.