Keep what gear you have.
Living in obx, you will use a light wind kite like maybe today or tomorrow, but not sunday. So stick with one brand so the bars can be used.
Think long term.
Once you get riding and staying up wind, light wind days will be an afterthought until you can gain a quiver for obx. Obx is my goto destination and i like waves area, sda and so on. So i ride Cabrinha and started with a 12m, then i got a 9, then a 15contra, then. 7, oh and just recently bought a ridiculously gigantic 17 contra that i bought as a session saver when at times in obx, i see the only people out were on these monsters....i bought new last year, tried to sell it, still wrapped in plastic never used. The 15 is big, the 17 is riducoulie.
So your question is a 13 or 14.. stick with one brand and look at building a quiver it is ok to buy good used equipment. Check some the shops there. I would say not real but perhaps nick at KHK,
or the newish shop in northern Avon. Also for lessons, see Barton at island surf and sail behind the dairy queen in waves for the best prices for lessons in the area. hatterasislandsurfshop.com. He has been there longer than sand.
Since you have a 10 the next size could be a big jump to the 13.
10 and 13 slingshots would be a start. You can always sell off to buy new to you stuff.
But if you are not in the water today or tomorrow, you def aint gonna be out on sunday with that 10. And if you arent kiting in cold water conditions right now, you have plenty of time to shop around for good used gear....try greenhatkiteboarding,
http://gulfportboardsports.com/ for good used gear.
Best advice, work on water launches, recovery, rescues, drift launches, body dragging and consistent tacks staying upwind.
Lastly, an ap i have been using is igetwind and windy for board sports. There you can enter your extremely fit 170lbs of kiting meanness into the profile and it will suggest kitesize......i feel a flamming of my ass coming on, KITE SIZE SUGGESTION, one can of course override.
Anyway, this was probably more guidance than you wanted but i am sure you appreciate any responses from the kite community, i was just watching the paint dry and counting my socks again. Cheers