I'd probably pick the MonAmour, for performance. To be honest, that's partly because I have a particular fondness for the 2A3 that has more to do with nostalgia. Other reasons are the larger plate choke inductance, the slightly simpler power supply circuit, the lower price of 2A3's, and the more attractive appearance.
For my own system, two MonAmours won't fit on my shelf so I"d probably wind up with Kaiju. (Incidentally, Kaiju does not use a dual mono power supply, but that is not important to me because the regulated driver and high-inductance plate chokes provide considerable isolation between channels.)