lighting - What flash unit should I choose for taking photos on a night boat cruise?

I'm going to a boat cruise tonight in London. With the following kit to choose from:

• Nikon D7000


• Nikkor 35mm f/1.8


• Nikkor 16-85mm f/3.5-5.6


• Nikon SB-400


• Yongnuo YN-465

I need to make a choice between the two flash units, as my bag doesn't have room for both. The SB-400 is small and compact, but has less maneuverability, and can only offer direct flash when shooting when there isn't a roof. The YN-465 on the other hand is a lot more powerful and 'rotate-able', and has a built-in bounce card to offer softer bounced light even when there is no roof, but its AF-assist lamp is quite weak and it can become impossible to focus when the lighting is really bad...

Anyone have tips/past experiences to offer please?

Also, I would appreciate any tips on best settings to use for flash as well, both for photos of people indoors, and portraits against dark cityscape background outside on the boat.

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_{This was originally part of another question on photo-SE.}