High guys, unfortunately Pat at Ison does not have a login so cannot help directly but has asked me to steer you in the right direction.
Not only have I ridden both sizes of phantom I also happen to be the NZ importer. I am 6ft and could ride either size, they feel longer than the spitfire.
First of all, picking bikes for a demo fleet is a difficult beast and we’re never going to get it 100%. However although sizing is important, demo’ing a bike is also about how the bike feels, the suspension, the angles, the pedalling.
I’d encourage you to try a medium and then make a decision from there, I’m sure it will help.