Flex or Three...
I must confess that the flex boys do seem to have more fun. Also the young ladies seem to think the flex wing boys are more daring.
When it comes to learning, I started lessons at the same time as another club member, me on 3-axis and him on flex. Weather impacted us both, as Watnochute says, but I ended up at double the number of hours flying even though he had more free time than I did. In autumn last year he switched to 3-axis to get the time in.
If you can get someone warm, sunny & with pretty low winds then I'd say flex-wing, but here in the UK with the weather then 3-axis first will give you a faster start.