I don't have experience with either choices. However, that being said, I have noticed:
Sim Audio I-I = really huge transformer, smaller power supply capacitance
Naim 5i = smaller transformer, larger amount of power supply capacitance
The huge transformer on the Sim Audio will compensate somewhat for the lesser capacitance. However, your Vandersteen 2C speakers are really easy to drive, there's only one dip down to about 6.7 ohms in the bass region -- mostly it's actually 7-8 ohms or higher. Both amps are 50 watts. All this together, I would lean towards the Naim having more power supply capacitance. In my experience, this always leads to better/smoother sound. You don't need a huge amount of brute force power here.
I read somewhere that Naim speaker wire must be used with any Naim amps for compatability and matching capacitance. Others might want to speak here, but this statement doesn't make sense to me. The Naim NACA5 is just 11.5 awg stranded copper. In your speaker wire choices, I would choose the DH Labs Q-10 speaker cable over everything else.