I need some extra info...at what windspeed, are you not able to plane any more (I'm a different weigth class - 6sqm is for me up to 25 knots).
But I can tell you this...
NCX (I have 7.0) is not bad, when fully powered it is stable and fast enough, when it gets really overpowered it is like a wild mustang (a lot of pressure on the back hand), manouvers are easy, not so simple to rigg, but still it is a no-cam sail.
Element (in my case 8.5)...very nice sail, good in low-end...I get planning with it and my 100kg in something like 11+ knots. It comfotable, stable even in high winds altough it is not a race sail...when it gets really overpowered it is not that nice anymore. Jiing and tacking is not a problem, cambers are only two...rigging is a bit more complicated but still easy compared to a full racing sails. On the water it gives me a very nice feeling.
In both of them mast plays a significant role, I used it at the beginning with a low carbon content mast (<35%) and it was not perfect in oth of them...but element wasa little less sensitive to the mast...at least due to my opinion.
I would go for elemet (early plannig, lots of low-end power)...NCX is too technical for beginers I think (but it works even when overpowered).
The other option would be a Gator (I haven't tried any by myself, but it is supposed to be a freemove sail...it means lot of low-end power, earli planninh, and easy handling)...about the size...I gues 7.5 would be ok, but I am not sure since I am different weight class
Have fun, whatever you pick...