I would opt for the Yagi antennas. I expect that they will do better in the multipath environment you're describing.
As far as vertical separation is concerned, be willing to experiment with moving either or both antennas up or down the mast in small steps. The UHF signals may be prone to 'hot' and 'cold' spots within a few inches of each other. The placement of the VHF antenna may have some influence on the phenomena. Ken Nist at
http://www.hdtvprimer.com/ discusses this in some detail.